Method and apparatus for measurement for inter-cell interference coordination in radio communication system

ABSTRACT

A user equipment, base station and method for performing a measurement in a wireless communication system are discussed. The method according to one embodiment includes receiving, from a first base station, information indicating a resource region for performing the measurement; performing the measurement for the resource region; and transmitting, to the first base station, a report for the measurement. The resource region is determined as a combination of at least one orthogonal frequency division multiplexing (OFDM) symbol and at least one downlink subframe.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.14/531,719 filed on Nov. 3, 2014, which is a continuation of U.S. patentapplication Ser. No. 13/509,748 filed on May 14, 2012 (now U.S. Pat. No.9,161,236 issued on Oct. 13, 2015), which is the National Phase ofPCT/KR2011/002155 filed on Mar. 29, 2011, which claims priority under 35U.S.C. §119(e) to U.S. Provisional Application No. 61/446,033 filed onFeb. 23, 2011, 61/405,215 filed on Oct. 21, 2010, 61/379,741 filed onSep. 3, 2010, and 61/318,758 filed on Mar. 29, 2010. The contents of allof these applications are hereby incorporated by reference as fully setforth herein in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a radio communication system, and moreparticularly, to a method and apparatus for measurement for inter-cellinterference coordination in a radio communication system.

Discussion of the Related Art

FIG. 1 illustrates a heterogeneous network wireless communicationssystem 100 including a macro base station and a micro base station. Inthe description of the present invention, the term “heterogeneousnetwork” refers to a network wherein a macro base station 110 and amicro base station 121 and 122 co-exist even when the same RAT (RadioAccess Technology) is being used.

A macro base station 110 refers to a general base station of a wirelesscommunication system having a broad coverage range and a hightransmission power. Herein, the macro base station 110 may also bereferred to a macro cell.

The micro base station 121 and 122 may also be referred to as a microcell, a pico cell, a femto cell, a home eNB (HeNB), a relay, and so on.More specifically, the micro base station 121 and 122 corresponds to asmall-sized version of the macro base station 110. Accordingly, themicro base station 121 and 122 may independently perform most of thefunctions of the macro base station. Herein, the micro base station 121and 122 may correspond to an overlay base station, which may beinstalled in an area covered by the macro base station, or to anon-overlay base station, which may be installed in a shadow area thatcannot be covered by the macro base station. As compared to the macrobase station 110, the micro base station 121 and 122 has a narrowercoverage range and a lower transmission power and may accommodate asmaller number of terminals (or user equipments).

A terminal 131 may directly receive services from the macro base station110 (hereinafter referred to as a macro-terminal). And, alternatively, aterminal 132 may directly receive services from the micro base station122 (hereinafter referred to as a micro-terminal). In some cases, aterminal 132 existing within the coverage area of the micro base station122 may receive services from the macro base station 110.

Depending upon whether or not the terminal has limited access, the microbase station may be categorized into two different types, the first typebeing a CSG (Closed Subscriber Group) micro base station, and the secondtype being an OA (Open Access) or OSC (Open Subscriber Group) micro basestation. More specifically, the CSG micro base station may serve onlyspecific terminals that are authorized, and the OSG micro base stationmay serve all types of terminals without any particular accesslimitations.

Meanwhile, the quality of a radio link between an eNB and a UE may bedegraded due to various factors. When the UE fails to receive a controlsignal from the eNB or the quality of a received signal is significantlydegraded, this may be defined as a Radio Link Failure (RLF). To handlethe RLF, the UE first identifies a problem at a physical layer andattempts to solve the physical layer problem. If the UE fails to recoverfrom the physical layer problem, the UE may transmit a connectionre-establishment request to the eNB, determining that an RLF has beendetected.

SUMMARY OF THE INVENTION

In the above-described heterogeneous network, when as user equipmentbeing served by a macro base station, due to an intense downlink signaltransmitted from a micro base station, an interference may occur in adownlink signal, which the macro user equipment receives from the macrobase station. Alternatively, a user equipment being served by a microbase station may receive an intense interference due to a downlinksignal of a macro base station. In order to prevent such interferencefrom occurring, for example, a method of using time or frequencyresource areas e.g., different subframes or different resource blocks)that can differentiate the micro base station from the macro basestation may be considered.

Even when such method for preventing inter-cell interference is applied,there may occur a case when, due to an interference from a micro basestation, a macro-user equipment existing within the coverage of a macrobase station detects a radio link failure (RLF) with the macro basestation and cannot communicate with the macro base station. For example,in case the micro base station performs transmission and receptionduring a specific section, if the macro-user equipment measures a signalfrom the macro base station during the corresponding specific section,despite the fact that there is no problem in the transmission andreception of the macro base station during the other sections excludingthe corresponding specific section, the macro-user equipment may becapable of detecting an RLF with the macro base station.

An object of the present invention is to provide a method and apparatusthat can enhance system efficiency by enabling a user equipment toaccurately perform downlink measurement, when an inter-cell interferencecoordination (ICIC) is being applied, by designating a resource area inwhich the user equipment may perform various downlink measurements(e.g., a measurement for an RLM (Radio Link Monitoring) with respect toRLF detection, a measurement for a Channel State Information (CSI)report, an interference measurement, an RRM (Radio Resource Management)measurement (measurements of Reference Signal Received Power (RSRP),Reference Signal Received Quality (RSRQ), Received Signal StrengthIndicator (RSSI), and so on)).

Another object of the present invention devised to solve the problemlies on a method for efficiently transmitting and receiving a signal ona backhaul link and an access link in a relay, if the relay performs amixture of an in-band operation and an out-band operation on multiplecarriers.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein,according to an embodiment of the present invention, a method forsupporting measurement of a User Equipment (UE) by a first base stationin a wireless communication system includes the steps of acquiringinformation of downlink subframe configuration of a second base station;determining measurement objects of downlink resource of the first basestation based on the downlink subframe configuration of the second basestation; transmitting information of the measurement objects to the UE;and receiving measurement result for the measurement objects from theUE.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein,according to another embodiment of the present invention, a method forperforming measurement by a User Equipment (UE) in a wirelesscommunication system includes receiving information of measurementobjects from a first base station; performing measurement for themeasurement objects; and transmitting measurement result to the firstbase station. Herein, the measurement objects may be determined fromdownlink resource of the first base station based on a downlink subframeconfiguration of a second base station.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein,according to yet another embodiment of the present invention, anapparatus for supporting measurement of a User Equipment (UE) includes areception module for receiving, uplink signals from the UE; atransmission module for transmitting downlink signals to the UE; and aprocessor for controlling transmission and reception of a first basestation through the transmission module and the reception module.Herein, the processor may be configured to acquire information ofdownlink subframe configuration of a second base station, to determinemeasurement objects of downlink resource of the first base station basedon the downlink subframe configuration of the second base station, totransmit information of the measurement objects to the UE through thetransmission module, and to receive measurement result for themeasurement objects from the UE through the reception module.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein,according to a further embodiment of the present invention, a UserEquipment (UE) for performing measurement includes a reception modulefor receiving downlink signals from a first base station; a transmissionmodule for transmitting uplink signals to the first base station; and aprocessor for controlling the UE including the transmission module andthe reception module. Herein, the processor may be configured to receiveinformation of measurement objects from a first base station through thereception module, to perform measurement for the measurement objects,and to transmit measurement result to the first base station through thetransmission module. Also, the measurement objects may be determinedfrom downlink resource of the first base station based on a downlinksubframe configuration of a second base station.

The following details may be equally and commonly applied to theembodiments of the present invention.

The measurement objects may include downlink resource of the first basestation not interfered by the second base station. Alternatively, themeasurement objects may include downlink resource of the first basestation constantly interfered by the second base station.

Information of the measurement objects may include at least one ofdownlink subframe, control region, data region, slot, OFDM symbol,resource block and antenna port of the first base station. Also, theinformation of the measurement objects may restrict a measurement regionfor the UE by one or a combination of downlink subframe, control region,data region, slot, OFDM symbol, resource block and antenna port of thefirst base station. Furthermore, the information of the measurementobjects may be transmitted through RRC (Radio Resource Control)signaling.

The downlink subframe configuration of the second base station mayinclude configuration of each of one or more downlink subframe of thesecond base station as normal subframe, ABS (Almost Blank Subframe),MBSFN (MulticastiBroadcast over Single Frequency Network) subframe orABS-with-MBSFN. And, the downlink subframe configuration of the secondbase station may include offset of a boundary of downlink subframe ofthe first base station and a boundary of downlink subframe of the secondbase station.

The measurement result may include measurement result for at least oneof RLM (Radio Link Monitoring) measurement, CSI (Channel StateInformation) measurement, interference measurement and RRM (RadioResource Management) measurement. And, the measurement result for RRMmeasurement includes RSRQ (Reference Signal Received Quality), the RSRQis measured by RSRP (Reference Signal Received Power) and RSSI (ReceivedSignal Strength Indicator), and the information of the measurementobjects for one or more of RSRQ, RSRP and RSSI restrict a measurementregion for the UE by all OFDM symbols in a downlink subframe of thefirst base station.

And, the first base station may be interfered by the second basestation.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

According, to an embodiment of the present invention, when an inter-cellinterference coordination (ICIC) is being applied, a method andapparatus enabling a user equipment to perform an accurate downlinkmeasurement may be provided, thereby enhancing system efficiency.

Additional advantages of the present application will be set forth inpart in the description which follows and in part will become apparentto those having ordinary skill in the art upon examination of thefollowing or may be learned from practice of the present application.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 illustrates a heterogeneous network wireless communicationsystem.

FIG. 2 illustrates a structure of a downlink wireless frame.

FIG. 3 illustrates a resource grid in a downlink slot.

FIG. 4 illustrates a exemplary structure of a downlink subframe.

FIG. 5 illustrates a structure of an uplink subframe.

FIG. 6 illustrates a block view showing the structure of a wirelesscommunication system having multiple antennae.

FIG. 7 illustrates CRS and DRS patterns that are defined in theconventional 3GPP LTE system.

FIG. 8 illustrates an uplink subframe structure including an SRS symbol.

FIG. 9 illustrates an example of realizing transmitter and receiverfunctions of an FDD mode relay station.

FIG. 10 illustrates a transmission of a user equipment from a relaystation and a downlink transmission of a relay station from a basestation.

FIG. 11 illustrates a coverage hole.

FIG. 12 to FIG. 18 illustrate examples of resource used for a downlinkmeasurement of a user equipment according to the present invention.

FIG. 19 illustrates a change in amount of interference according tointerference cell subframe settings.

FIG. 20 illustrates a downlink measurement method of a user equipmentwith respect to an inter-cell interference coordination and a method forsupporting the same according to an embodiment of the present invention.

FIG. 21 illustrates a base station device and a user equipment deviceaccording to a preferred embodiment of the present invention.

FIG. 22 illustrates measurement objects that may be signaled through ameasConfig information element (IE) included in aRRCConnectionReconfiguration message according to an example of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered to be optional factors on thecondition that there is no additional remark. If required, theindividual constituent components or characteristics may not be combinedwith other components or characteristics. Also, some constituentcomponents and/or characteristics may be combined to implement theembodiments of the present invention. The order of operations to bedisclosed in the embodiments of the present invention may be changed toanother. Some components or characteristics of any embodiment may alsobe included in other embodiments, or may be replaced with those of theother embodiments as necessary.

The embodiments of the present invention are disclosed on the basis of adata communication relationship between a base station and a terminal.In this case, the base station is used as a terminal node of a networkvia which the base station can directly communicate with the terminal.Specific operations to be conducted by the base station in the presentinvention may also be conducted by an upper node of the base station asnecessary.

In other words, it will be obvious to those skilled in the art thatvarious operations for enabling the base station to communicate with theterminal in a network composed of several network nodes including thebase station will be conducted by the base station or other networknodes other than the base station. The term “Base Station (BS)” may bereplaced with a fixed station, Node-B, eNode-B (eNB), or an access pointas necessary. The term “relay” may be replaced with a Relay Node (RN) ora Relay Station (RS). The term “terminal” may also be replaced with aUser Equipment (UE), a Mobile Station (MS), a Mobile Subscriber Station(MSS) or a Subscriber Station (SS) as necessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for the convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to another format within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention and theimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802 system, a 3rd Generation Project Partnership (3GPP) system, a 3GPPLong Term Evolution (LTE) system, and a 3GPP2 system. In particular, thesteps or parts, which are not described to clearly reveal the technicalidea of the present invention, in the embodiments of the presentinvention may be supported by the above documents. All terminology usedherein may be supported by at least one of the above-mentioneddocuments.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, CDMA (CodeDivision Multiple Access), FDMA (Frequency Division Multiple Access),TDMA (Time Division Multiple Access), OFDMA (Orthogonal FrequencyDivision Multiple Access), SC-FDMA (Single Carrier Frequency DivisionMultiple Access), and the like. The CDMA may be embodied with radiotechnology such as UTRA (Universal Terrestrial Radio Access) orCDMA2000. The TDMA may be embodied with radio technology such as GSM(Global System for Mobile communications)/GPRS (General Packet RadioService)/EDGE (Enhanced Data Rates for GSM. Evolution). The OFDMA may beembodied with radio technology such as Institute of Electrical andElectronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE802-20, and E-UTRA (Evolved UTRA). The UTRA is a part of the UNITS(Universal Mobile Telecommunications System). The 3GPP (3rd GenerationPartnership Project) LTE (long term evolution) is a part of the E-UMTS(Evolved UMTS), which uses E-UTRA. The 3GPP LTE employs the OFDMA indownlink and employs the SC-FDMA in uplink. The LTE-Advanced (LTE-A) isan evolved version of the 3GPP LTE. WiMAX can be explained by an IEEE802.16e (WirelessMAN-OFDMA Reference System) and an advanced IEEE802.16m (WirelessMAN-OFDMA Advanced System). For clarity, the followingdescription focuses on the 3GPP LTE and 3GPP LTE-A system. However,technical features of the present invention are not limited thereto.

The structure of a downlink radio frame will be described with referenceto FIG. 2.

In a cellular Orthogonal Frequency Division Multiplexing (OFDM) radiopacket communication system, uplink/downlink data packet transmission isperformed in subframe units. One subframe is defined as a predeterminedtime interval including a plurality of OFDM symbols. The 3GPP LTEstandard supports a type 1 radio frame structure applicable to FrequencyDivision Duplex (FDD) and a type 2 radio frame structure applicable toTime Division Duplex (TDD).

FIG. 2(a) is a diagram showing the structure of the type 1 radio frame.A downlink radio frame includes 10 subframes, and one subframe includestwo slots in time domain. A time required for transmitting one subframeis defined in a Transmission Time Interval (TTI). For example, onesubframe may have a length of 1 ms and one slot may have a length of 0.5ms. One slot may include a plurality of OFDM symbols in time domain andinclude a plurality of Resource Blocks (RBs) in frequency domain. Sincethe 3GPP LTE system uses OFDMA in downlink, the OFDM symbol indicatesone symbol duration. The OFDM symbol may be called a SC-FDMA symbol or asymbol duration. A RB is a resource allocation unit and includes aplurality of contiguous subcarriers in one slot.

The number of OFDM symbols included in one slot may be changed accordingto the configuration of a Cyclic Prefix (CP). The CP includes anextended CP and a normal CP. For example, if the OFDM symbols areconfigured by the normal CP, the number of OFDM symbols included in oneslot may be seven. If the OFDM symbols are configured by the extendedCP, the length of one OFDM symbol is increased, the number of OFDMsymbols included in one slot is less than that of the case of the normalCP. In case of the extended CP, for example, the number of OFDM symbolsincluded in one slot may be six. If a channel state is instable, forexample, if a User Equipment (UE) moves at a high speed, the extended CPmay be used in order to further reduce interference between symbols.

In case of using the normal CP, since one slot includes seven OFDMsymbols, one subframe includes 14 OFDM symbols. At this time, the firsttwo or three OFDM symbols of each subframe may be allocated to aPhysical Downlink Control Channel (PDCCH) and the remaining OFDM symbolsmay be allocated to a Physical Downlink Shared Channel (PDSCH).

FIG. 2(b) is a diagram showing the structure of the type 2 radio frame.The type 2 radio frame includes two half frames, each of which includesfive subframes, a downlink pilot time slot (DwPTS), a guard period (GP),and an uplink pilot time slot (UpPTS). One of these subframes includestwo slots. The DwPTS is used for initial cell, search, synchronizationand channel estimation at a user equipment. The UpPTS is used forchannel estimation and uplink transmission synchronization of the userequipment. The guard period is to remove interference occurring in anuplink due to multi-path delay of a downlink signal between the uplinkand a downlink. Meanwhile, one subframe includes two slots regardless ofa type of the radio frame.

The structure of the radio frame is only exemplary. Accordingly, thenumber of subframes included in the radio frame, the number of slotsincluded in the subframe or the number of symbols included in the slotmay be changed in various manners.

FIG. 3 is a diagram showing a resource grid in a downlink slot. Althoughone downlink slot includes seven OFDM symbols in a time domain and oneRB includes 12 subcarriers in a frequency domain in the figure, thepresent invention is not limited thereto. For example, in case of anormal Cyclic Prefix (CP), one slot includes 7 OFDM symbols. However, incase of an extended CP, one slot, includes 6 OFDM symbols. Each elementon the resource grid is referred to as a resource element. One RBincludes 12×7 resource elements. The number NDL of RBs included in thedownlink slot is determined based on a downlink transmission bandwidth.The structure of the uplink slot may be equal to the structure of thedownlink slot.

FIG. 4 is a diagram showing the structure of a downlink subframe. Amaximum of three OFDM symbols of a front portion of a first slot withinone subframe corresponds to a control region to which a control channelis allocated. The remaining OFDM symbols correspond to a data region towhich a Physical Downlink Shared Channel (PDSCH) is allocated. Examplesof the downlink control channels used in the 3GPP LTE system include,for example, a Physical Control Format Indicator Channel (PCFICH), aPhysical Downlink Control Channel (PDCCH), a Physical Hybrid automaticrepeat request Indicator Channel (PHICH), etc. The PCFICH is transmittedat a first OFDM symbol of a subframe, and includes information about thenumber of OFDM symbols used to transmit the control channel in thesubframe. The PHICH includes a HARQ ACK/NACK signal as a response ofuplink transmission. The control information transmitted through thePDCCH is referred to as Downlink Control Information (DCI). The DCIincludes uplink or downlink scheduling information or an uplink transmitpower control command for a certain UE group. The PDCCH may includeresource allocation and transmission format of a Downlink Shared Channel(DL-SCH), resource allocation information of an Uplink Shared Channel(UL-SCH), paging information of a Paging Channel (PCH), systeminformation on the DL-SCH, resource allocation of an higher layercontrol message such as a Random Access Response (RAR) transmitted onthe PDSCH, a set of transmit power control commands for an individualUEs in a certain UE group, transmit power control information,activation of Voice over IP (VoIP), etc. A plurality of PDCCHs may betransmitted within the control region. The UE may monitor the pluralityof PDCCHs. The PDCCHs are transmitted, on an aggregation of one orseveral consecutive control channel elements (CCEs). The CCE is alogical allocation unit used to provide the PDCCHs at a coding ratebased on the state of a radio channel. The CCE corresponds to aplurality of resource element groups. The format of the PDCCH and thenumber of available bits are determined based on a correlation betweenthe number of CCEs and the coding rate provided by the CCEs. The basestation determines a PDCCH format according to a DCI to be transmittedto the UE, and attaches a Cyclic Redundancy Check (CRC) to controlinformation. The CRC is masked with a Radio Network Temporary Identifier(RNTI) according to an owner or usage of the PDCCH. If the PDCCH is fora specific UE, a cell-RNTI (C-RNTI) of the UE may be masked to the CRC.Alternatively, if the PDCCH is for a paging message, a paging indicatoridentifier (P-RNTI) may be masked to the CRC. If the PDCCH is for systeminformation (more specifically, a system information block (SIB)), asystem information identifier and a system information RNTI (SI-RNTI)may be masked to the CRC. To indicate a random access response that is aresponse for transmission of a random access preamble of the UE, arandom access-RNTI (RA-RNTI) may be masked to the CRC.

FIG. 5 is a diagram showing the structure of an uplink frame. The uplinksubframe may be divided into a control region and a data region in afrequency domain. A Physical Uplink Control Channel (PUCCH) includinguplink control information is allocated to the control region. APhysical uplink Shared Channel (PUSCH) including user data is allocatedto the data region. In order to maintain single carrier property, one UEdoes not simultaneously transmit the PUCCH and the PUSCH. The PUCCH forone UE is allocated to a RB pair in a subframe, RBs belonging to the RBpair occupy different subcarriers with respect to two slots. Thus, theRB pair allocated to the PUCCH is “frequency-hopped” at a slot boundary.

Modeling of Multi-Input Multi-Output (MIMO) System

FIG. 6 is a diagram showing the configuration of a radio communicationsystem having multiple antennas.

As shown in FIG. 6(a), if the number of transmission antennas isincreased to NT and the number of reception antennas is increased to NR,a theoretical channel transmission capacity is increased in proportionto the number of antennas, unlike the case where a plurality of antennasis used in only a transmitter or a receiver. Accordingly, it is possibleto improve a transfer rate and to remarkably improve frequencyefficiency. As the channel transmission capacity is increased, thetransfer rate may be theoretically increased by a product of a maximumtransfer rate R0 upon using a single antenna and a rate increase ratioRi.R _(i)=min(N _(T) ,N _(R))  Equation 1

For example, in an MIMO system using four transmission antennas and fourreception antennas, it is possible to theoretically acquire a transferrate which is four times that of a single antenna system. After theincrease in the theoretical capacity of the MIMO system was proved inthe mid-1990s, various technologies of substantially improving a datatransfer rate have been actively developed up to now. In addition,several technologies are already applied to the various radiocommunication standards such as the third-generation mobilecommunication and the next-generation wireless local area network (LAN).

According to the researches into the MIMO antenna up to now, variousresearches such as researches into information theory related to thecomputation of the communication capacity of a MIMO antenna in variouschannel environments and multiple access environments, researches intothe model and the measurement of the radio channels of the MIMO system,and researches into space-time signal processing technologies ofimproving transmission reliability and transmission rate have beenactively conducted.

The communication method of the MIMO system will be described in moredetail using mathematical modeling. In the above system, it is assumedthat N_(T) transmission antennas and N_(R) reception antennas arepresent.

In transmitted signals, if the N_(T) transmission antennas are present,the number of pieces of maximally transmittable information is N_(T).The transmitted information may be expressed as follows.s=└s ₁ ,s ₂ , . . . ,s _(N) _(T) ┘^(T)  Equation 2

The transmitted information S₁, S₂, . . . , S_(N) _(T) may havedifferent transmit powers. If the respective transmit powers are P₁, P₂,. . . , P_(N) _(T) , the transmitted information with adjusted powersmay be expressed as follows.ŝ=[ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T) =[P ₁ s ₁ ,P ₂ s ₂ , . . . ,P_(N) _(T) s _(N) _(T) ]^(T)  Equation 3

In addition, Ŝ may be expressed using a diagonal matrix P of thetransmit powers as follows.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

Consider that the N_(T) actually transmitted signals x₁, x₂, . . . ,x_(N) _(T) are configured by applying a weight matrix W to theinformation vector Ŝ with the adjusted transmit powers. The weightmatrix W serves to appropriately distribute the transmitted informationto each antenna according to a transport channel state, etc. x₁, x₂, . .. , x_(N) _(T) may be expressed by using the vector X as follows.

$\begin{matrix}{x = {\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix} = {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1\; N_{T}} \\w_{21} & w_{22} & \ldots & w_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

where, w_(ij) denotes a weight between an i-th transmission antenna andj-th information. W is also called a precoding matrix.

In received signals, if the N_(R) reception antennas are present,respective received signals y₁, y₂, . . . , y_(N) _(R) of the antennasare expressed as follows.y=[y ₁ ,y ₂ , . . . y _(N) _(R) ]^(T)  Equation 6

If channels are modeled in the MIMO radio communication system, thechannels may be distinguished according to transmission/receptionantenna indexes. A channel from the transmission antenna j to thereception antenna i is denoted by h_(ij). In h_(ij), it is noted thatthe indexes of the reception antennas precede the indexes of thetransmission antennas in view of the order of indexes.

FIG. 6(b) is a diagram showing channels from the N_(T) transmissionantennas to the reception antenna i. The channels may be combined andexpressed in the form of a vector and a matrix. In FIG. 6(b), thechannels from the N_(T) transmission antennas to the reception antenna imay be expressed as follows.hd i ^(T) =[h _(i1) ,h _(i2) , . . . ,h _(iN) _(T) ]  Equation 7

Accordingly, all the channels from the N_(T) transmission antennas tothe N_(R) reception antennas may be expressed as follows.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1\; N_{T}} \\h_{21} & h_{22} & \ldots & h_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}} & {{Equation}\mspace{14mu} 8}\end{matrix}$

An Additive White Gaussian Noise (AWGN) is added to the actual channelsafter a channel matrix H. The AWGN n₁, n₂, . . . , n_(N) _(R) added tothe N_(T) transmission antennas may be expressed as follows.n=[n ₁ ,n ₂ , . . . ,n _(N) _(R) ]^(T)  Equation 9

Through the above-described mathematical modeling, the received signalsmay be expressed as follows,

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {{{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1\; N_{T}} \\h_{21} & h_{22} & \ldots & h_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}} = {{Hx} + n}}}} & {{Equation}\mspace{14mu} 10}\end{matrix}$

The number of rows and columns of the channel matrix H indicating thechannel state is determined by the number of transmission and receptionantennas. The number of rows of the channel matrix H is equal to thenumber N_(R) of reception antennas and the number of columns thereof isequal to the number N_(T) of transmission antennas. That is, the channelmatrix H is an N_(R)×N_(T) matrix.

The rank of the matrix is defined by the smaller of the number of rowsor columns, which is independent from each other. Accordingly, the rankof the matrix is not greater than the number of rows or columns. Therank rank (

) of the channel matrix

is restricted as follows.rank(H)≦min(N _(T) ,N _(R))  Equation 11

When the matrix is subjected to Eigen value decomposition, the rank maybe defined by the number of Eigen values excluding 0. Similarly, whenthe matrix is subjected to singular value decomposition, the rank may bedefined by the number of singular values excluding 0. Accordingly, thephysical meaning of the rank in the channel matrix may be a maximumnumber of different transmittable information in a given channel.

Reference Signal (RS)

In a radio communication system, since packets are transmitted through aradio channel, a signal may be distorted during transmission. In orderto enable a reception side to correctly receive the distorted signal,distortion of the received signal should be corrected using channelinformation. In order to detect the channel information, a method oftransmitting a signal, of which both the transmission side and thereception side are aware, and detecting channel information using adistortion degree when the signal is received through a channel ismainly used. The above signal is referred to as a pilot signal or areference signal (RS).

When transmitting and receiving data using multiple antennas, thechannel states between the transmission antennas and the receptionantennas should be detected in order to correctly receive the signal.Accordingly, each transmission antenna has an individual RS.

A downlink RS includes a Common RS (CRS) shared among all UEs in a celland a Dedicated RS (DRS) for only a specific-UE. It is possible toprovide information for channel estimation and demodulation using suchRSs.

The reception side (UE) estimates the channel state from the CRS andfeeds back an indicator associated with channel quality, such as aChannel Quality Indicator (CQI), a Precoding Matrix Index (PMI) and/or aRank Indicator (RI), to the transmission side (eNodeB). The CRS may bealso called a cell-specific RS. Alternatively, an RS associated with thefeedback of Channel State information (CSI) such as CQI/PMI/RI may beseparately defined as a CSI-RS.

The DRS may be transmitted through REs if data demodulation on a PDSCHis necessary. The UE may receive the presence/absence of the DRS from ahigher layer and receive information indicating that the DRS is validonly when the PDSCH is mapped. The DRS may be also called a UE-specificRS or a Demodulation RS (DMRS).

FIG. 7 is a diagram showing a pattern of CRSs and DRSs mapped on adownlink RB defined in the existing 3GPP LTE system (e.g., Release-8).The downlink RB as a mapping unit of the RSs may be expressed in unitsof one subframe on a time domain×12 subcarriers on a frequency domain.That is, on the time axis, one RB has a length of 14 OFDM symbols incase of the normal CP (FIG. 7(a)) and has a length of 12 OFDM symbols incase of the extended CP (FIG. 7(b)).

FIG. 7 shows the locations of the RSs on the RB in the system in whichthe eNodeB supports four transmission antennas. In FIG. 7, ResourceElements (REs) denoted by “0”, “1”, “2” and “3” indicate the locationsof the CRSs of the antenna port indexes 0, 1, 2 and 3, respectively. InFIG. 7, the RE denoted by “D” indicates the location of the DRS.

Hereinafter, the CRS will be described in detail.

The CRS is used to estimate the channel of a physical antenna and isdistributed over the entire band as an RS which is able to be commonlyreceived by all UEs located within a cell. The CRS may be used for CSIacquisition and data demodulation.

The CRS is defined in various formats according to the antennaconfiguration of the transmission side (eNodeB). The 3GPP LTE (e.g.,Release-8) system supports various antenna configurations, and adownlink signal transmission side (eNodeB) has three antennaconfigurations such as a single antenna, two transmission antennas andfour transmission antennas. If the eNodeB performs single-antennatransmission, RSs for a single antenna port are arranged. If the eNodeBperforms two-antenna transmission, RSs for two antenna ports arearranged using a Time Division Multiplexing (TDM) and/or FrequencyDivision Multiplexing (FDM) scheme. That is, the RSs for the two antennaports are arranged in different time resources and/or differentfrequency resources so as to be distinguished from each other. Inaddition, if the eNodeB performs four-antenna transmission, RSs for fourantenna ports are arranged using the TDM/FDM scheme. The channelinformation estimated by the downlink signal reception side (LIE)through the CRSs may be used to demodulate data transmitted using atransmission scheme such as single antenna transmission, transmitdiversity, closed-loop spatial multiplexing, open-loop spatialmultiplexing, or Multi-User MIMO (MU-MIMO).

If multiple antennas are supported, when RSs are transmitted from acertain antenna port, the RSs are transmitted at the locations of theREs specified according to the RS pattern and any signal is nottransmitted at the locations of the REs specified for another antennaport.

The rule of mapping the CRSs to the RBs is defined by Equation 12.

$\begin{matrix}{{k = {{6\; m} + {( {v + v_{shift}} ){mod}\; 6}}}{l = \{ {{{\begin{matrix}{0,{N_{symb}^{DL} - 3}} & {{{if}\mspace{14mu} p} \in \{ {0,1} \}} \\1 & {{{if}\mspace{14mu} p} \in \{ {2,3} \}}\end{matrix}m} = 0},1,\ldots\mspace{14mu},{{{2 \cdot N_{RB}^{DL}} - {1m^{\prime}}} = {{m + N_{RB}^{\max,{DL}} - {N_{RB}^{DL}v}} = \{ {{\begin{matrix}0 & {{{if}\mspace{14mu} p} = {{0\mspace{14mu}{and}\mspace{14mu} l} = 0}} \\3 & {{{if}\mspace{14mu} p} = {{0\mspace{14mu}{and}\mspace{14mu} l} \neq 0}} \\3 & {{{if}\mspace{14mu} p} = {{1\mspace{14mu}{and}\mspace{14mu} l} = 0}} \\0 & {{{if}\mspace{14mu} p} = {{1\mspace{14mu}{and}\mspace{14mu} l} \neq 0}} \\{3( {n_{s}{mod}\; 2} )} & {{{if}\mspace{14mu} p} = 2} \\{3 + {3( {n_{s}{mod}\; 2} )}} & {{{if}\mspace{14mu} p} = 3}\end{matrix}v_{shift}} = {N_{ID}^{cell}{mod}\; 6}} }}} }} & {{Equation}\mspace{14mu} 12}\end{matrix}$

In Equation 12, k denotes a subcarrier index, l denotes a symbol index,and p denotes an antenna port index. N_(symb) ^(DL) denotes the numberof OFDM symbols of one downlink slot, N_(RB) ^(DL) denotes the number ofRBs allocated to the downlink, n_(s) denotes a slot index, and N_(ID)^(cell) denotes a cell ID. mod indicates a modulo operation. Thelocation of the RS in the frequency domain depends on a value V_(shift).Since the value V_(shift) depends on the cell ID, the location of the RShas a frequency shift value which varies according, to the cell.

In detail, in order to increase channel estimation performance throughthe CRSs, the locations of the CRSs in the frequency domain may beshifted so as to be changed according, to the cells. For example, if theRSs are located at an interval of three subcarriers, the RSs arearranged on 3k-th subcarriers in one cell and arranged on (3k+1)-thsubcarriers in the other cell. In view of one antenna port, the RSs arearranged at an interval of 6 REs (that is, interval of 6 subcarriers) inthe frequency domain and are separated from REs, on which RSs allocatedto another antenna port are arranged, by 3 REs in the frequency domain.

In addition, power boosting is applied to the CRSs. The power boostingindicates that the RSs are transmitted using higher power by bringing(stealing) the powers of the REs except for the REs allocated for theRSs among the REs of one OFDM symbol.

In the time domain, the RSs are arranged from a symbol index (l=0) ofeach slot as a starting point at a constant interval. The time intervalis differently defined according to the CP length. The RSs are locatedon symbol indexes 0 and 4 of the slot in case of the normal CP and arelocated on symbol indexes 0 and 3 of the slot in case of the extendedCP. Only RSs for a maximum of two antenna ports are defined in one OFDMsymbol. Accordingly, upon four-transmission antenna transmission, theRSs for the antenna ports 0 and 1 are located on the symbol indexes 0and 4 (the symbol indexes 0 and 3 in case of the extended CP) of theslot and the RSs for the antenna ports 2 and 3 are located on the symbolindex 1 of the slot. The frequency locations of the RSs for the antennaports 2 and 3 in the frequency domain are exchanged with each other in asecond slot.

In order to support spectrum efficiency higher than that of the existing3GPP LTE (e.g., Release-8) system, a system (e.g., an LTE-A system)having the extended antenna configuration may be designed. The extendedantenna configuration may have, for example, eight transmissionantennas. In the system having the extended antenna configuration, UEswhich operate in the existing antenna configuration needs to besupported, that is, backward compatibility needs to be supported.Accordingly, it is necessary to support a RS pattern according to theexisting antenna configuration and to design as new RS pattern for anadditional antenna configuration. If CRSs for the new antenna ports areadded to the system having the existing antenna configuration, RSoverhead is rapidly increased and thus data transfer rate is reduced. Inconsideration of these problems, in an LTE-A (Advanced) system which isan evolution version of the 3GPP LTE system, separate RSs (CSI-RSs) formeasuring the CSI for the new antenna ports may be used.

Hereinafter, the DRS will be described in detail.

The DRS (or UE-specific RS) is used to demodulate data. A precodingweight used for a specific UE upon multi-antenna transmission is alsoused in an RS without change so as to estimate an equivalent channel, inwhich a transfer channel and the precoding weight transmitted from eachtransmission antenna are combined, when the UE receives the RSs.

The existing 3GPP LTE system (e.g., Release-8) supportsfour-transmission antenna transmission as a maximum and the DRS for Rank1 beamforming is defined. The DRS for Rank 1 beamforming is also denotedby the RS for the antenna port index 5. The rule of the DRS mapped onthe RBs is defined by Equations 13 and 14. Equation 13 is for the normalCP and Equation 14 is for the extended CP.

$\begin{matrix}{{k = {{( k^{\prime} ){mod}\; N_{sc}^{RB}} + {N_{sc}^{RB} \cdot n_{PRB}}}}{k^{\prime} = \{ {{\begin{matrix}{{4\; m^{\prime}} + v_{shift}} & {{{if}\mspace{14mu} l} \in \{ {2,3} \}} \\{{4\; m^{\prime}} + {( {2 + v_{shift}} ){mod}\; 4}} & {{{if}\mspace{14mu} l} \in \{ {5,6} \}}\end{matrix}l} = \{ {{\begin{matrix}3 & {l^{\prime} = 0} \\6 & {l^{\prime} = 1} \\2 & {l^{\prime} = 2} \\5 & {l^{\prime} = 3}\end{matrix}l^{\prime}} = \{ {{{\begin{matrix}{0,1} & {{{if}\mspace{14mu} n_{s}\mspace{11mu}{mod}\; 2} = 0} \\{2,3} & {{{if}\mspace{14mu} n_{s}\mspace{11mu}{mod}\; 2} = 1}\end{matrix}m^{\prime}} = 0},1,\ldots\mspace{14mu},{{{3\; N_{RB}^{PDSCH}} - {1v_{shift}}} = {N_{ID}^{cell}{mod}\; 3}}} } } }} & {{Equation}\mspace{14mu} 13} \\{{k = {{( k^{\prime} ){mod}\; N_{sc}^{RB}} + {N_{sc}^{RB} \cdot n_{PRB}}}}{k^{\prime} = \{ {{\begin{matrix}{{3\; m^{\prime}} + v_{shift}} & {{{if}\mspace{14mu} l} = 4} \\{{3\; m^{\prime}} + {( {2 + v_{shift}} ){mod}\; 3}} & {{{if}\mspace{14mu} l} = 1}\end{matrix}l} = \{ {{\begin{matrix}4 & {l^{\prime} \in \{ {0,2} \}} \\1 & {l^{\prime} = 1}\end{matrix}l^{\prime}} = \{ {{{\begin{matrix}0 & {{{if}\mspace{14mu} n_{s}\mspace{11mu}{mod}\; 2} = 0} \\{1,2} & {{{if}\mspace{14mu} n_{s}\mspace{11mu}{mod}\; 2} = 1}\end{matrix}m^{\prime}} = 0},1,\ldots\mspace{14mu},{{{4\; N_{RB}^{PDSCH}} - {1v_{shift}}} = {N_{ID}^{cell}{mod}\; 3}}} } } }} & {{Equation}\mspace{14mu} 14}\end{matrix}$

In Equations 13 and 14, k denotes a subcarrier index, l denotes a symbolindex, and p denotes an antenna port index. N_(SC) ^(RB) denotes theresource block size in the frequency domain and is expressed by thenumber of subcarriers. n_(PRB) denotes a physical resource block number.N_(RB) ^(PDSCH) denotes the bandwidth of the RB of the PDSCHtransmission. n_(s) denotes a slot index, and N_(ID) ^(cell) denotes acell ID. mod indicates a modulo operation. The location of the RS in thefrequency domain depends on a value V_(shift). Since the value V_(shift)depends on the cell ID, the location of the RS has a frequency shiftvalue which varies according to the cell.

In the LTE-A system which is the evolution version of the 3GPP LTEsystem, high-order MIMO, multi-cell transmission, evolved MU-MIMO or thelike is considered. In order to support efficient RS management and adeveloped transmission scheme, DRS-based data demodulation isconsidered. That is, separately from the DRS (antenna port, index 5) forRank 1 beamforming defined in the existing 3GPP LTE (e.g., Release-8)system, DRSs for two or more layers may be defined in order to supportdata transmission through the added antenna.

Cooperative Multi-Point (CoMP)

According to the improved system performance requirements of the 3GPPLTE-A system, CoMP transmission/reception technology (may be referred toas co-MIMO, collaborative MIMO or network MIMO) is proposed. The CoMPtechnology can increase the performance of the UE located on a cell edgeand increase average sector throughput.

In general, in a multi-cell environment in which a frequency reusefactor is 1, the performance of the UE located on the cell edge andaverage sector throughput may be reduced due to Inter-Cell Interference(ICI). In order to reduce the ICI, in the existing LTE system, a methodof enabling the UE located on the cell edge to have appropriatethroughput and performance using a simple passive method such asFractional Frequency Reuse (FFR) through the UE-specific power controlin the environment restricted by interference is applied. However,rather than decreasing the use of frequency resources per cell, it ispreferable that the ICI is reduced or the UE reuses the ICI as a desiredsignal. In order to accomplish the above object, a CoMP transmissionscheme may be applied.

The CoMP scheme applicable to the downlink may be largely classifiedinto a Joint Processing (JP) scheme and a CoordinatedScheduling/Beamforming (CS/CB) scheme.

In the JP scheme, each point (eNodeB) of a CoMP unit may use data. TheCoMP unit refers to a set of eNodeBs used in the CoMP scheme. The JPscheme may be classified into a joint transmission scheme and a dynamiccell selection scheme.

The joint transmission scheme refers to a scheme for transmitting aPDSCH from a plurality of points (a part or the whole of the CoMP unit).That is, data transmitted to a single UE may be simultaneouslytransmitted from a plurality of transmission points. According to thejoint transmission scheme, it is possible to coherently ornon-coherently improve the quality of the received signals and toactively eliminate interference with another UE.

The dynamic cell selection scheme refers to a scheme for transmitting aPDSCH from one point (of the CoMP unit). That is, data transmitted to asingle UE at a specific time is transmitted from one point and the otherpoints in the cooperative unit at that time do not transmit data to theUE. The point for transmitting the data to the UE may be dynamicallyselected.

According to the CS/CB scheme, the CoMP units may cooperatively performbeamforming of data transmission to a single UE. Although only a servingcell transmits the data, user scheduling/beamforming may be determinedby the coordination of the cells of the CoMP unit.

In uplink, coordinated multi-point reception refers to reception of asignal transmitted by coordination of a plurality of geographicallyseparated points. The CoMP scheme applicable to the uplink may beclassified into Joint Reception (JR) and CoordinatedScheduling/Beamforming (CS/CB).

The JR scheme indicates that a plurality of reception points receives asignal transmitted through a PUSCH, the CS/CB scheme indicates that onlyone point receives a PUSCH, and user scheduling/beamforming isdetermined by the coordination of the cells of the CoMP unit.

Sounding RS (SRS)

An SRS is used for enabling an eNodeB to measure channel quality so asto perform frequency-selective scheduling on the uplink and is notassociated with uplink data and/or control information transmission.However, the present invention is not limited thereto and the SRS may beused for improved power control or supporting of various start-upfunctions of UEs which are not recently scheduled. Examples of thestart-up functions may include, for example, initial Modulation andCoding Scheme (MCS), initial power control for data transmission, timingadvance, and frequency-semi-selective scheduling (scheduling forselectively allocating frequency resources in a first slot of a subframeand pseudo-randomly hopping to another frequency in a second slot).

In addition, the SRS may be used for downlink channel qualitymeasurement on the assumption that the radio channel is reciprocalbetween the uplink and downlink. This assumption is particularly validin a Time Division Duplex (TDD) system in which the same frequency bandis shared between the uplink and the downlink and is divided in the timedomain.

The subframe through which the SRS is transmitted by a certain UE withinthe cell is indicated by cell-specific broadcast signaling. 4-bitcell-specific “srsSubframeConfiguration” parameter indicates 15 possibleconfigurations of the subframe through which the SRS can be transmittedwithin each radio frame. By such configurations, it is possible toprovide adjustment flexibility of SRS overhead according to a networkarrangement scenario. The remaining one (sixteenth) configuration of theparameters indicates the switch-off of the SRS transmission within thecell and is suitable for a serving cell for serving high-rate UEs.

As shown in FIG. 8, the SRS is always transmitted on a last SC-FDMAsymbol of the configured subframe. Accordingly, the SRS and aDemodulation RS (DMRS) are located on different SC-FDMA symbols. PUSCHdata transmission is not allowed on the SC-FDMA symbol specified for SRStransmission and thus sounding overhead does not approximately exceed 7%even when it is highest (that is, even when SRS transmission symbols arepresent in all subframes).

Each SRS symbol is generated by the basic sequence (random sequence orZadoff-Ch (ZC)-based sequence set) with respect to a given time unit andfrequency band, and all UEs within the cell use the same basic sequence.At this time, the SRS transmission of the plurality of UEs within thecell in the same time unit and the same frequency band is orthogonallydistinguished by different cyclic shifts of the base sequence allocatedto the plurality of UEs. The SRS sequences of different cells can bedistinguished by allocating different basic sequences to respectivecells, but the orthogonality between the different basic sequences isnot guaranteed.

Relay Node (RN)

A RN may be considered for, for example, enlargement of high data ratecoverage, improvement of group mobility, temporary network deployment,improvement of cell edge throughput and/or provision of network coverageto a new area.

A RN forwards data transmitted or received between the eNodeB and theUE, two different links (backhaul link and access link) are applied tothe respective carrier frequency bands having different attributes. TheeNodeB may include a donor cell. The RN is wirelessly connected to aradio access network through the donor cell.

The backhaul link between the eNodeB and the RN may be represented by abackhaul downlink if downlink frequency bands or downlink subframeresources are used, and may be represented by a backhaul uplink ifuplink frequency bands or uplink subframe resources are used. Here, thefrequency band is resource allocated in a Frequency Division Duplex(FDD) mode and the subframe is resource allocated in a Time DivisionDuplex (TDD) mode. Similarly, the access link between the RN and theUE(s) may be represented by an access downlink if downlink frequencybands or downlink subframe resources are used, and may be represented byan access uplink if uplink frequency bands or uplink subframe resourcesare used.

The eNodeB must have functions such as uplink reception and downlinktransmission and the UE must have functions such as uplink transmissionand downlink reception. The RN must have all functions such as backhauluplink transmission to the eNodeB, access uplink reception from the UE,the backhaul downlink reception from the eNodeB and access downlinktransmission to the UE.

FIG. 9 is a diagram showing an example of implementing transmission andreception functions of a FDD-mode RN. The reception function of the RNwill now be conceptually described. A downlink signal received from theeNodeB is forwarded to a Fast Fourier Transform (FFT) module 912 througha duplexer 911 and is subjected to an OFDMA baseband reception process913. An uplink signal received from the UE is forwarded to a FFT module922 through a duplexer 921 and is subjected to a Discrete FourierTransform-spread-OFDMA (DFT-s-OFDMA) baseband reception process 923. Theprocess of receiving the downlink signal from the eNodeB and the processof receiving the uplink signal from the UE may be simultaneouslyperformed. The transmission function of the RN will now be described.The uplink signal transmitted to the eNodeB is transmitted through aDFT-s-OFDMA baseband transmission process 933, an Inverse FFT (IFFT)module 932 and a duplexer 931. The downlink signal transmitted to the UEis transmitted through an OFDM baseband transmission process 943, anIFFT module 942 and a duplexer 941. The process of transmitting theuplink signal to the eNodeB and the process of transmitting the downlinksignal to the UE may be simultaneously performed. In addition, theduplexers shown as functioning in one direction may be implemented byone bidirectional duplexer. For example, the duplexer 911 and theduplexer 931 may be implemented by one bidirectional duplexer and theduplexer 921 and the duplexer 941 may be implemented by onebidirectional duplexer. The bidirectional duplexer may branch into theIFFT module associated with the transmission and reception on a specificcarrier frequency band and the baseband process module line.

In association with the use of the band (or the spectrum) of the RN, thecase where the backhaul link operates in the same frequency band as theaccess link is referred to as “in-band” and the case where the backhaullink and the access link operate in different frequency bands isreferred to as “out-band”. In both the in-band case and the out-bandcase, a UE which operates according, to the existing LTE system (e.g.,Release-8), hereinafter, referred to as a legacy UE, must be able to beconnected to the donor cell.

The RN may be classified into a transparent RN or a non-transparent RNdepending on whether or not the UE recognizes the RN. The term“transparent” indicates that the UE cannot recognize whethercommunication with the network is performed through the RN and the term“non-transparent” indicates that the UE recognizes whether communicationwith the network is performed through the RN.

In association with the control of the RN, the RN may be classified intoa RN configured as a part of the donor cell or a RN for controlling thecell.

The RN configured as the part of the donor cell may have a RN ID, butdoes not have its cell identity. When at least a part of Radio ResourceManagement (RRM) of the RN is controlled by the eNodeB to which thedonor cell belongs (even when the remaining parts of the RRM are locatedon the RN), the RN is configured as the part of the donor cell.Preferably, such an RN can support a legacy UE. For example, examples ofsuch an RN include various types of relays such as smart repeaters,decode-and-forward relays, L2 (second layer) relays and Type-2 relays.

In the RN for controlling the cell, the RN controls one or severalcells, unique physical layer cell identities are provided to the cellscontrolled by the RN, and the same RRM mechanism may be used. From theviewpoint of the UE, there is no difference between access to the cellcontrolled by the RN and access to the cell controlled by a generaleNodeB. Preferably, the cell controlled by such an RN may support alegacy UE. For example, examples of such an RN include self-backhaulingrelays, L3 (third layer) relays. Type-1 relays and Type-1a relays.

The Type-1 relay is an in-band relay for controlling a plurality ofcells, which appears to be different from the donor cell, from theviewpoint of the UE. In addition, the plurality of cells has respectivephysical cell IDs (defined in the LTE Release-8) and the RN may transmitits synchronization channel, RSs, etc. In a single-cell operation, theUE may directly receive scheduling information and HARQ feedback fromthe RN and transmit its control channel (Scheduling Request (SR), CQI,ACK/NACK, etc.) to the RN. In addition, a legacy UE (a UE which operatesaccording to the LTE Release-8 system) regards the Type-1 relay as alegacy eNodeB (an eNodeB which operates according to the LTE Release-8system). That is, the Type-1 relay has backward compatibility. The UEswhich operates according to the LTE-A system regard the Type-1 relay asan eNodeB different from the legacy eNodeB, thereby achievingperformance improvement.

The Type-1a relay has the same characteristics as the above-describedType-1 relay except that it operates as an out-band relay. The Type-1arelay may be configured so as to minimize or eliminate an influence ofthe operation thereof on an L1 (first layer) operation.

The Type-2 relay is an in-band relay and does not have a separatephysical cell ID. Thus, a new cell is not established. The Type-2 relayis transparent to the legacy UE and the legacy UE does not recognize thepresence of the Type-2 relay. The Type-2 relay can transmit a PDSCH, butdoes not transmit at least a CRS and a PDCCH.

In order to enable the RN to operate as the in-band relay, someresources in a time-frequency space must be reserved for the backhaullink so as not to be used for the access link. This is called resourcepartitioning.

The general principle of the resource partitioning in the RN will now bedescribed. The backhaul downlink and the access downlink may bemultiplexed on one carrier frequency using a Time Division Multiplexing(TDM) scheme (that is, only one of the backhaul downlink or the accessdownlink is activated in a specific time). Similarly, the backhauluplink and the access uplink may be multiplexed on one carrier frequencyusing the TDM scheme (that is, only one of the backhaul uplink or theaccess uplink is activated in a specific time).

The multiplexing of the backhaul link using a FDD scheme indicates thatbackhaul downlink transmission is performed in a downlink frequency bandand the backhaul uplink transmission is performed in an uplink frequencyband. The multiplexing of the backhaul link using the TDD schemeindicates that the backhaul downlink transmission is performed in adownlink subframe of the eNodeB and the RN and the backhaul uplinktransmission is performed in an uplink subframe of the eNodeB and theRN.

In the in-band relay, for example, if the backhaul downlink receptionfrom the eNodeB and the access downlink transmission to the UE aresimultaneously performed in a predetermined frequency band, the signaltransmitted from the transmitter of the RN may be received by thereceiver of the RN and thus signal interference or RF jamming may occurin the RF front end of the RN. Similarly, if the access uplink receptionfrom the UE and the backhaul uplink transmission to the eNodeB aresimultaneously performed in a predetermined frequency band, signalinterference may occur in the RF front end of the RN. Accordingly, it isdifficult to implement the simultaneous transmission and reception inone frequency band at the RN unless the received signal and thetransmitted signal are sufficiently separated (for example, unless thetransmission antennas and the reception antennas are sufficientlyseparated form each other (for example, on the ground or under theground) in terms of geographical positions).

As a method for solving the signal interference, the RN operates so asnot to transmit a signal to the UE while a signal is received from thedonor cell. That is, a gap may be generated in the transmission from theRN to the UE and any transmission from the RN to the UE (including thelegacy UE) may not be performed. Such a gap may be set by configuring aMulticast Broadcast Single Frequency Network (MBSFN) subframe (see FIG.10). In FIG. 10, a first subframe 1010 is a general subframe, in which adownlink (that is, access downlink) control signal and data istransmitted from the RN to the UE, and a second subframe 1020 is anMBSFN subframe, in which a control signal is transmitted from the RN tothe UE in a control region 1021 of the downlink subframe, but any signalis not transmitted from the RN to the UE in the remaining region 1022 ofthe downlink subframe. Since the legacy UE expects the transmission ofthe PDCCH in all downlink subframes (that is, the RN needs to enable thelegacy UEs within its own area to receive the PDCCH in every subframe soas to perform a measurement function), for the correct operation of thelegacy UEs, it is necessary to transmit the PDCCH in all the downlinksubframes. Accordingly, even on the subframe (the second subframe 1020))set for the transmission of the downlink (that is, the backhauldownlink) from the eNodeB to the RN, the RN needs to transmit the accessdownlink in first N (N=1, 2 or 3) OFDM symbol intervals of the subframe,without receiving the backhaul downlink. Since the PDCCH is transmittedfrom the RN to the UE in the control region 1021 of the second subframe,it is possible to provide backward compatibility to the legacy UE servedby the RN. While any signal is not transmitted from the RN to the UE inthe remaining region 1022 of the second subframe, the RN may receive thesignal transmitted from the eNodeB. Accordingly, the resourcepartitioning disables the in-band RN to simultaneously perform theaccess downlink transmission and the backhaul downlink reception.

The second subframe 1022 using the MBSFN subframe will now be describedin detail. The control region 1021 of the second subframe may be a RNnon-hearing interval. The RN non-hearing interval refers to an intervalin which the RN does not receive a backhaul downlink signal andtransmits an access downlink signal. This interval may be set to 1, 2 or3 OFDM lengths as described above. The RN performs the access downlinktransmission to the UE in the RN non-hearing interval 1021 and performsthe backhaul downlink reception from the eNodeB in the remaining region1022. At this time, since the RN cannot simultaneously perform thetransmission and reception in the same frequency band, it takes acertain length of time to switch the RN from the transmission mode tothe reception mode. Accordingly, it is necessary to set a guard time(GT) to switch the RN from the transmission mode to the reception modein a first portion of the backhaul downlink reception region 1022.Similarly, even when the RN receives the backhaul downlink from theeNodeB and transmits the access downlink to the UE, a guard time (GT) toswitch the RN from the reception mode to the transmission mode may beset. The length of the guard time may be set to values of the timedomain, for example, values of k (k≧1) time samples Ts or one or moreOFDM symbol lengths. Alternatively, if the backhaul downlink subframesof the RN are consecutively set or according to a predetermined subframetiming alignment relationship, the guard time of a last portion of thesubframes may not be defined or set. Such a guard time may be definedonly in the frequency domain set for the transmission of the backhauldownlink subframe, in order to maintain backward compatibility (thelegacy UE cannot be supported if the guard time is set in the accessdownlink interval). The RN can receive a PDCCH and a PDSCH from theeNodeB in the backhaul downlink reception interval 1022 except for theguard time. Such PDCCH and the PDSCH are physical channels dedicated forRN and thus may be represented by a R-PDCCH (Relay-PDCCH) and a R-PDSCH(Relay-PDSCH).

RLF-Related Operation

An RLF may occur between an eNB and a UE. The REF means a state in whichthe quality of a radio link between the eNB and the UE is degraded andthus signal transmission and reception is difficult between them. Adescription will be given below of a procedure for detecting an RLF andsearching for a new radio link.

In the 3GPP LTE system, Radio Resource Control (RRC) state between aneNB and a UE is divided into RRC_CONNECTED state and RRC_IDLE state. Inthe RRC_CONNECTED state, an RRC connection has been established betweenthe eNB and the UE and thus the UE can transmit data to and receive datafrom the eNB. When the RRC connection is released between the UE and theeNB, this state is called the RRC_IDLE state.

An RLF-related operation involves (1) detection of a physical layerproblem in the RRC_CONNECTED state, (2) recovery from the physical layerproblem, and (3) RLF detection.

(1) Upon receipt of as many consecutive “out-of-sync” indications as apredetermined value N310 from a lower layer, the UE activates a T310timer. “Out-of-sync” indications is an event occurring when the UEmeasures signals from a serving eNB and the quality of measured channelfalls below a predetermined level. Here, the channel quality may bedetermined by SNR (Signal-to-Noise Ratio) measured using Cell-specificReference Signal (CRS) from the downlink signals. Further, the lowerlayer (i.e. the physical layer) may provide an “out-of-sync” indicationto a higher layer, when demodulation of a received PDCCH is impossibleor the Signal-to-Interference plus Noise Ratio (SINR) of the PDCCH islow. N310 and T310 are higher-layer parameters that may be preset.

(2) Upon receipt of as many consecutive “in-sync” indications as apredetermined value N311 while the T310 timer is running, the UE stopsthe T310 timer. N311 is a higher-layer parameter that may be predefined.

(3) Upon expiration of the T310 timer, the UE starts a connectionre-establishment procedure, determining that an RLF has been detected.The expiration of the T310 timer implies that the T310 timer has reacheda predetermined time T310 without stopping in the middle. In theconnection re-establishment procedure, the UE transmits an RRCconnection re-establishment request to the eNB, receives an RRCconnection re-establishment message from the eNB, and then transmits anRRC connection re-establishment completion message to the eNB. Fordetails of the RLF-related operation, section 5.3.11 of the 3GPPstandard document, TS36.331 may be referred to.

As stated before, the RLF process is a process of searching for a newlink, when the link state between a transmitter and a receiver keepsdegraded during activating an internal timer. Because it is difficult topredict the state of the link (Uu link) between the eNB and the UE inthe 3GPP LTE system, it is determined whether an RLF has been detectedusing parameters such as N310, N311 and T310.

Operation of Measurement for ICIC

Referring back to FIG. 1, description will be made on a case where aninter-cell interference of a micro base station with respect to adownlink to a macro-user equipment occurs from a macro base station. Forexample, it is assumed that the micro base station (122) corresponds toa CSG cell allowing access only to a specific user equipmentAdditionally, it is assumed that the user equipment (132) corresponds toa macro-user equipment that is served by the macro base station (110).More specifically, it is also assumed that the user equipment (132) isnot included in the CSG of the micro base station (122). In this case,the user equipment (132) is positioned within the coverage of the microbase station (122). However, since the user equipment (132) is incapableof accessing the corresponding micro base station (122), the userequipment (132) may perform transmission and reception to and from amacro base station 110, which is located at a more remote location. As aresult, in performing downlink reception, the user equipment (132)eventually receives an intense interference from the micro base station(122).

A variety of methods for controlling such inter-cell interference may beconsidered. For example, a case assuming that a macro base stationreceives an interference from the micro base station will now bedescribed. As a method for controlling inter-cell interference, a methodof reducing the influence on uplink/downlink quality between the macrobase station and the macro-user equipment, by chronologically/spatiallyshifting uplink/downlink transmission resource between the micro basestation and the micro-user equipment, and a method of reducing theinfluence by performing, a puncturing process in a downlink of the microbase station on essential parts (e.g., CRS) of the downlink signals ofthe macro base station, wherein the essential parts of the downlinksignals are used for maintaining a radio link with the macro-userequipment, may be considered. Additionally, as a method for reducinginfluence of the micro base station on the macro base station, a methodenabling the micro base station to perform transmission only during aspecific section (e.g., an odd-numbered subframe within the time or apartial RB within the frequency) may also be considered.

However, even when such inter-cell interference coordinating methods areapplied, an RLF between the macro base station and the macro-userequipment may still occur. For example, in case the macro-user equipmentis positioned at a remote location from the macro base station, and incase the macro-user equipment is served by the macro base station, dueto a strong interference from as micro base station neighboring themacro-user equipment, the macro-user equipment may detect that a radiolink quality between the macro-user equipment and the macro base stationis extremely low. In case an RLF occurs, the macro-user equipmentdetermines that the radio link with the macro base station is notsuitable for transmission and reception. And, accordingly, themacro-user equipment performs a procedure for searching a new adequatecell. Most particularly, among the above-described inter-cellinterference coordinating methods, in case the micro base stationapplies a method of performing transmission only during a specificsection, even if there is no operational problem in the macro basestation during the other sections excluding the corresponding specificsection, there may lie a problem in that the user equipment detects anRLF and searches for another cell. In this case, although the coverageof the micro base station and its surrounding area belong to thecoverage of the macro base station, a problem may occur whereintransmission and reception cannot be performed between the macro userequipment and the macro base station. As described above, an area wherethe transmission and reception of the macro base station is obstructed(or interrupted) by the micro base station may be expressed as acoverage hole.

FIG. 11 illustrates a coverage hole. As shown in FIG. 11, in casemultiple micro base stations exist within the coverage of a macro basestation, due to an intense interference of the micro base station, acoverage hole, wherein the transmission and reception between themacro-user equipment and the macro base station, may occur.

As described above, in case the conventional inter-cell interferencecoordination (ICIC) method is applied, and if the user equipmentmeasures the radio link by using an identical method as the conventionalmethod, problems such as the occurrence of the above-described coveragehole cannot be resolved. Therefore, in order to prevent the occurrenceof a coverage hole caused by an RLF, which may occur due to inter-cellinterference, and to allow the user equipment to measure channel qualityof a radio link with more accuracy, when performing CSI measurement, RRMmeasurement, and so on, the present invention proposes a method ofdesignating a resource area that corresponds to a measurement object,when the user equipment measures a downlink channel quality from thebase station to the user equipment. According to the present invention,by having the user equipment perform signaling on a downlink resource(time resource, frequency resource, and/or space resource) that is to bemeasured, the user equipment may correctly measure the quality of thewireless resource, even when the inter-cell interference is largelyapplied, problems such as the occurrence of unnecessary RLF may beprevented. The various embodiments proposed in the present invention maybe effective in cases wherein, for example, CSG cells exist within thecoverage of the macro base station.

In the following description, it will be assumed that 2 cells receiveand transmit interference to and from one another for clarity of thedescription. Hereinafter, a method enabling a cell receiving theinterference (also referred to as a victim cell) to signal a downlinkresource to a user equipment that is served by the victim cell itself(also referred to as a victim UE) will be described. Herein, thedownlink resource is to be measured by the corresponding user equipment.Also, the cell causing the interference may also be expressed as aninterfering cell or an aggressor cell. For example, in case of a networkwherein the macro base station co-exists with a femto-cell base station,the macro user equipment being located within the coverage of thefemto-cell base station may become the victim cell, and the femto cellmay become the aggressor cell. Alternatively, in case of a networkwherein the macro base station co-exists with a pico-cell base station,the user equipment being served by the pico-cell base station within theextended area of the pico-cell base station may become the victim userequipment, and the macro base station may become the interfering cell.

In the above-described example, description is made under the assumptionthat the macro base station is the victim cell and that the micro basestation is the interfering cell, for simplicity. However, theembodiments of the present invention may also be applied in cases otherthan the case of the above-described example. For example, when themicro-user equipment measures a downlink signal from the micro basestation, the same principle described in the present invention may beapplied to a case where an intense interference caused by the macro basestation exists. Alternatively, the same principle described in thepresent invention may be applied to a case where an interference existsbetween 2 macro cells. More specifically, it will be apparent that, incase an interference can occur between 2 random cells, variousembodiments of the present invention can be applied.

Additionally, as a case wherein the principle of the present inventionis applicable, an exemplary case of preventing unnecessary RLF frombeing detected by the victim user equipment, when the effect of theinter-cell interference is large, has been given to describe theabove-mentioned example. The present invention will not be limited onlyto the above-described example. The basic principle of designating adownlink measurement resource may enable the victim user equipment toaccurately and efficiently perform downlink measurement, when theinter-cell interference exists, and the basic principle of the presentinvention may also be applied to a case when a resource for measuring adownlink from a neighboring cell, which is adjacent to a specific userequipment, is designated. More specifically, it will be specified thatthe method of designating a downlink measurement resource proposed inthe present invention can be applied to various downlink measurementschemes of the user equipment.

In other words, the downlink measurement of a user equipment according,to the present invention refers to a collective concept including RLM(Radio Link Monitoring) for preventing RLF, a measurement for ChannelState Information (CSI) reporting, interference measurement, RRM (RadioResource Management) measurement, and so on. The RRM measurement mayinclude, for example, the measurements of Reference Signal ReceivedPower (RSRP), Reference Signal Received Quality (RSRQ), Received SignalStrength Indicator (RSSI), and so on.

Designation of Downlink Measurement Resource

The present invention proposes a method of designating resource timeresource, frequency resource, and/or space resource) areas, which areused for the measurement performed by the user equipment, as follows.The designation of the resource that is to be used for the measurementprocess may be notified to the user equipment by a physical layersignaling or a higher-level layer signaling from the base station.

(1) Designating Subtropics

The user equipment may designate downlink measurement to be performedonly in a specific subframe. The specific subframe, for example, may bedesignated as a subframe receiving no downlink transmission from aninterfering cell. Additionally, the designated specific subframe maycorrespond to one or more subframes.

(2) Designating Control Regions/Data Regions

The user equipment may designate downlink measurement to be performedonly in a PDCCH region (or control region) or a PDSCH region (or dataregion) of a random downlink subframe.

(3) Designating Slots

The user equipment may designate downlink measurement to be performedonly in a specific slot of a random downlink subframe.

(4) Designating OFDM Symbols

The user equipment may designate downlink measurement to be performedonly in a specific PFDM symbol of a random downlink subframe. Herein,the designated OFDM symbol may correspond to one or more OFDM symbols.

(5) Designating Resource Blocks (RBs)

The user equipment may designate downlink measurement to be performedonly in a specific RB within a frequency resource. Herein, thedesignated specific RB may correspond to one or more RBs.

Alternatively, the user equipment may designate a specific RB by using abitmap method. Also, in order to reduce signaling overhead, signalingmay be performed in bundle units (bundles of multiple RBs). Furthermore,signaling may also be performed by using an offset value of a start RBindex and an end RB index.

(6) Designating Transmission Antenna Ports

The user equipment may designate downlink measurement to be performed byusing only a reference signal being transmitted from a specific antennaport. For example, the user equipment may designate downlink channelquality to be measured by using only a CRS (RE marked as “0” in FIG. 7)allocated to antenna port 0, or the user equipment may designatedownlink channel quality to be measured by using only a CRS (REs markedas “0” and “1” in FIG. 7) allocated to antenna ports 0 and 1.

The above-described examples (1) to (6) on the designation of resourcesused for the downlink measurement performed by the user equipment may beapplied independently or in combination. For example, signaling may bemade so that downlink measurement can be performed only in a specific RBwithin the control region of a specific downlink subframe.Alternatively, signaling may be made so that downlink channel qualitycan be measured by using only the CRS of antenna ports 0 and 1 of thecontrol region within a specific downlink subframe. Further, acombination of (1) designating subframe and (3) designating OFDM symbolmay be applied, resulting in that downlink measurement may be performedfor certain OFDM symbol(s) in certain downlink subframe(s) or for allOFDM symbols in certain downlink subframe(s). Furthermore, when multiplecombinations are being applied, the user equipment may designate adownlink measurement area in resource element (RE) units. Accordingly,in case the effect of the inter-cell interference is large, the basestation may designate a downlink resource area (e.g., a resource areathat is not influenced by an interference caused by another cell may bedesignated) in which the user equipment is to measure channel quality.Thus, the radio link between the base station and the user equipment canbe accurately measured. (For example, the radio link may be maintainedby preventing the RLF from being unnecessarily detected.) Alternatively,by performing downlink measurement in a portion having no downlinksignal transmitted from an interfering cell, which is adjacent to thevictim user equipment, the victim user equipment may be capable ofaccurately calculating the CSI or may be capable of accuratelyperforming RRM measurement, such as RSRP, RSSI, RSRQ, and so on.

Detailed examples of designating a resource that is to be used for thedownlink measurement performed by the user equipment according to thepresent invention will now be described with reference to FIG. 12 andFIG. 13.

The embodiment of the present invention shown in FIG. 12 describes acase wherein downlink measurement is designated to be performed only inan even-numbered subframe of a channel bandwidth of 1.4 MHz (a whole RB(6RB) is used within the bandwidth of 1.4 MHz), and wherein downlinkmeasurement is designated to be performed by using only the CRS locatedin the control region (PDCCH region) within each RB (or the CRS existingin OFDM symbol indexes 0 and 1). Herein, the antenna port through whichthe user equipment is to perform downlink measurement may also beadditionally designated. For example, in case downlink measurement isdesignated to be performed only with respect to antenna port 0,signaling may be made so that downlink measurement using only 2 REs (REsmarked as R0 in FIG. 12) for each RB can be performed.

The embodiment shown in FIG. 13(a) and FIG. 13(b) is similar to theembodiment shown in FIG. 12 in that the downlink measurement isperformed only in an even-numbered subframe. However, the embodimentshown in FIG. 13(a) and FIG. 13(b) describes a case wherein the downlinkmeasurement is designated to be performed only with respect to 4RB ofthe 1.4 MHz channel bandwidth (6RB). This may correspond, for example,to a case where a victim user equipment designates a downlink frequencyresource that is to be measured, when the victim user equipment uses thelower 4RB of the 6RB so as to perform transmission, and when theinterfering cell uses the upper 2RB of the 6RB so as to performtransmission (or including a case when the interfering cell uses 2RB ormore so as to perform transmission). Herein, the embodiment of FIG.13(a) shows a case where downlink measurement is performed only withrespect to a CRS allocated to antenna port 1 in a data region (PDSCHregion) of a downlink subframe. The embodiment of FIG. 13(b) shows acase where downlink measurement is performed only with respect to a CRSallocated to antenna port 1 in a second slot of a downlink subframe.Alternatively, the designation of downlink measurement resources shownin FIG. 13(a) and FIG. 13(b) may also be expressed as a designation of aCRS RE existing in a specific OFDM symbol of a downlink subframe. Forexample, the case shown in FIG. 13(a) may correspond to a case where thedownlink measurement is designated to be performed only on the CRSallocated to antenna port 1 at OFDM symbol indexes 4, 7, and 11. And,the case shown in FIG. 13(b) may correspond to a case where the downlinkmeasurement is designated to be performed only on the CRS allocated toantenna port 1 at OFDM symbol indexes 7 and 11.

In the above-described example, a case where the user equipment uses theCRS so as to measure the downlink channel quality has been described, inorder to clearly describe the principle of the present invention.However, the scope of the present invention will not be limited only tothe above-described example. The above-described details may be equallyapplied to a process of signaling a resource area in which variousdownlink measurements (measurement for RLM, CSI measurement,interference measurement, RRM measurement) are to be performed, thevarious downlink measurements being performed by the use equipment. Morespecifically, according to the present invention, signaling may beperformed on a specific time resource (e.g., subframe, controlregion/data region, slot or OFDM symbol), a specific frequency resource(e.g., RB), and/or a specific space resource (e.g., antenna port) withrespect to which the various downlink measurements are to be performedby the user equipment.

Designating Resource for Downlink CSI Measurement

Hereinafter, a method of designating a downlink measurement resourcecorresponding to a case when the user equipment computes channel stateinformation (CSI) according to an embodiment of the present inventionwill be described in detail.

As one of many enhanced inter-cell interference coordination (enhancedICIC or eICIC) methods, an MBSFN subframe may be configured from a cellcausing interference (interfering cell). As a general rule, the MBSFNsubframe corresponds to a subframe for MBMS (multimedia Broadcast andMulticast Service), and MBMS refers to a service transmitting the samesignal from multiple cells at the same time. A downlink subframe that isconfigured as the MBSFN subframe may transmit CRS only from an OFDMsymbol position, which transmits the control channel, and the CRS is nottransmitted from the data region. Furthermore, it is assumed that aboundary of a downlink subframe of an interfering cell and a boundary ofa downlink subframe of a cell receiving interference (victim cell) arealigned. Accordingly, the cell receiving the inference (victim cell)receives (or is influenced by) the interference caused by the CRS of theinterfering cell only in the control region (PDCCH region), and thevictim cell does not receive (or is not influenced by) the interferencecaused by the CRS of the interfering cell in the data region (PDSCHregion). In this case, when the victim user equipment computes andreports the CSI for PDSCH transmission from the victim cell, theinfluence of the CRS interference caused by the interfering cell is notrequired to be considered. Also, in order to enable the victim userequipment to compute and report a more accurate CSI, the CSI may becomputed by using only the CRS located in the PDSCH region (i.e., CRSreceived from the victim cell) of the corresponding subframe (downlinksubframe of the victim cell being aligned with a downlink subframeconfigured by the interfering cell as an MBSFN subframe).

In order to enable the user equipment to perform such downlinkmeasurement operations, the base station may designate and signal thecorresponding user equipment to perform downlink measurement only in thedata region. More specifically, the above-described method number (2),which designates the resource that is used for the downlink measurementprocess, may be applied.

Additionally, when the user equipment computes the CSI, an SINR of areceived signal may be measured. Herein, in order to compute the SINR, asignal element and an interference element (or interference and noiseelements) should be estimated. As described above, in case theinterfering cell configures a specific downlink subframe as an MBSFNsubframe and in case the interfering cell does not transmit data and CRSin the data region (or in case the interfering cell transmits a nullresource element (Null RE)), in a downlink subframe of the victim cellthat is aligned with the specific downlink subframe of the interferingcell, the victim user equipment may perform interference estimation byusing only the CRS of the data region. (For example, the victim userequipment extracts a CRS of the data region within the downlink subframethat is received from the victim cell. Then, the victim user equipmentmay measure the remaining elements as interference elements). At thispoint, the process of measuring the signal elements may be performed byusing the downlink signals of both the data region and the controlregion, or the process of measuring the signal elements may be performedby using only the downlink signal of a specific region (e.g., dataregion).

Although an example of the interfering cell configuring an MBSFNsubframe has been given to describe the above-described embodiment ofthe present invention, the present invention will not be limited only tothe example given herein. In other words, the same principle may beapplied to normal subframes, ABSs (Almost Blank Subframes), andABS-with-MBSFN. More specifically, in order to allow the user equipmentto measure downlink CSI with more accuracy, the base station maydesignate and signal a resource area in which the downlink measurementis to be performed. Herein, the ABS refers to a case wherein the CRS istransmitted only from the control region and data region of the downlinksubframe, and wherein the PDCCH and the PDSCH are not transmitted.However, even in the ABS, downlink channels, such as PBCH, PSS, SSS, andso on, and downlink signals may be transmitted. Furthermore,ABS-with-MBSFN refers to a case wherein even the CRS of the data regionis not transmitted from the above-described ABS.

In the above-described examples, it is described that the CRS is usedfor the downlink CSI measurement. However, the principle of the presentinvention may also be equally applied in a case of using CSI-RS in orderto measure downlink CSI. More specifically, in order to enable the userequipment to measure downlink CSI with more accuracy based upon a CSI-RStransmitted from the base station, which has an extended antennaconfiguration, the base station may designate and signal a resource areain which the downlink measurement process is to be performed.

According to another embodiment of the present invention, descriptionwill be made on a method of designating a downlink measurement resourceof a victim user equipment, in case the number of transmission antennaeof the interfering cell is smaller than the number of transmissionantennae of the victim cell for in case the number of transmissionantennae of the interfering cell is limited). In this case, the victimuser equipment may perform interference measurement by using the CRS ofan antenna port that is not used by the interfering cell. Accordingly,the victim user equipment may apply the measured result to CSIcomputation. Most particularly, in case the interfering cell configuresa specific downlink subframe as the ABS, and when the victim userequipment uses the CSI-RS to compute the downlink CSI, the interferencemay be measured by using the CRS of an antenna port that is not used bythe interfering cell in a downlink subframe from which the CSI-RS is nottransmitted.

In the above-described examples, a case wherein the user equipmentmeasures the downlink CSI has been described in order to clearlydescribe the principle of the present invention. However, the scope ofthe present invention will not be limited only to the case described inthe example presented herein. The above-described details may also beequally applied to a process of signaling a resource area, whereinvarious downlink measurement processes (measurement for RLM, CSImeasurement, interference measurement, RRM measurement), which arepreformed by the user equipment, are to be performed. More specifically,according to the present invention, signaling may be performed on aspecific time resource (e.g., subframe, control region/data region, slotor OFDM symbol), a specific frequency resource (e.g., RB), and/or aspecific space resource (e.g., antenna port) with respect to which thevarious downlink measurements are to be performed by the user equipment.

Detailed examples of designating a downlink measurement respective tointer-cell interference coordination according to the present inventionwill now be described with reference to FIG. 14 to FIG. 18.

FIG. 14 illustrates an example of designating a resource, on whichdownlink measurement is to be performed, from a victim cell beinginfluenced by interference to a victim user equipment, in case downlinksubframe boundaries of two cells exchanging interference are identical,and in case CRS transmission resource elements of two cells coincide. Inthe example shown in FIG. 14, the antenna configuration of the victimcell corresponds to 4 transmission antennae, and the antennaconfiguration of the interfering cell corresponds to 2 transmissionantenna. Additionally, the example of FIG. 14 shows a case wherein thedownlink subframe of the interfering cell is configured as an ABS.Accordingly, a collision may occur between the CRS being allocated toantenna ports 0 and 1 of the interfering cell and the CRS beingallocated to antenna ports 0 and 1 of the victim cell. In this case, thevictim cell may signal the victim user equipment to perform downlinkmeasurement by using only the CRS being allocated to antenna ports 2 and3 in the downlink subframe.

FIG. 15 illustrates an example of designating a resource, on whichdownlink measurement is to be performed, from a victim cell beinginfluenced by interference to a victim user equipment, in case downlinksubframe boundaries of two cells exchanging interference are identical,and in case CRS transmission resource elements of two cells coincide. Inthe example shown in FIG. 15, the antenna configuration of both thevictim cell and the interfering cell corresponds to 4 transmissionantennae. Additionally, the example of FIG. 15 shows a case wherein thedownlink subframe of the interfering cell is configured as an MBSFNsubframe. Accordingly, a collision may occur between the CRS of thecontrol region of a downlink subframe of the interfering cell and theCRS of the control region of a downlink subframe of the victim cell. Inthis case, the victim cell may signal the victim user equipment toperform downlink measurement by using, only the CRS of the data regionin the downlink subframe.

FIG. 16 illustrates an example of designating a resource, on whichdownlink measurement is to be performed, from a victim cell beinginfluenced by interference to a victim user equipment, in case downlinksubframe boundaries of two cells exchanging interference are shifted toan offset of 3 OFDM symbols. By shifting the subframe boundaries of twocells to an offset of 3 OFDM symbols, the PDCCH region of the victimuser equipment may be protected or CRC collision may be prevented. Inthe example shown in FIG. 16, the antenna configuration of both thevictim cell and the interfering cell corresponds to 4 transmissionantennae. Additionally, the example of FIG. 16 shows a case wherein thedownlink subframe of the interfering cell is configured as an MBSFNsubframe. Accordingly, a portion of the PDSCH region of the victim cellmay be influenced by an interference caused by the PDCCH and CRS of theinterfering cell. In this case, the victim cell may signal the victimuser equipment to perform downlink measurement by using only the CRS ofa second slot in the downlink subframe. Alternatively, if the last 3OFDM symbols of a previous subframe of the interfering cell (first 3OFDM symbols in the bottom drawing of FIG. 16) do not influence thePDCCH region of the victim cell at all, the victim cell may designatethe victim user equipment to also use the CRS of the PDCCH region so asto downlink measurement.

FIG. 17 illustrates an example of designating a resource, on whichdownlink measurement is to be performed, from a victim cell beinginfluenced by interference to a victim user equipment, in case downlinksubframe boundaries of two cells exchanging interference are identical,and in case CRS transmission resource elements of two cells coincide. Inthe example shown in FIG. 17, the antenna configuration of both thevictim cell and the interfering cell corresponds to 4 transmissionantennae. Additionally, the example of FIG. 17 shows a case wherein thedownlink subframe of the interfering cell is configured as anABS-with-MBSFN subframe. Accordingly, a collision may occur between theCRS of the control region of a downlink subframe of the interfering celland the CRS of the control region of a downlink subframe of the victimcell. In this case, the victim cell may signal the victim user equipmentto perform downlink measurement by using only the CRS of the data regionin the downlink subframe.

FIG. 18 illustrates an example of designating a resource, on whichdownlink measurement is to be performed, from a victim cell beinginfluenced by interference to a victim user equipment, in case downlinksubframe boundaries of two cells exchanging interference are shifted to2 OFDM symbols. By shifting the subframe boundaries of two cells, thePDCCH region of the victim user equipment may be protected or CRCcollision may be prevented. In the example shown in FIG. 18, theinterfering cell configures two consecutive subframes as ABS. In theexample shown in FIG. 18, the downlink measurement may be designated tobe performed in a downlink subframe symbol of the victim cell thatcoincides with the ABS of the interfering cell. For example, in theexample of FIG. 18, in case the entire nth downlink subframe of thevictim cell is included in the ABS section of the interfering cell, theCRS of the entire subframe may be used for the measurement process inthe nth downlink subframe of the victim cell. Additionally, the CRS ofthe PDCCH region (or the CRS of an available OFDM symbol) may be usedfor the measurement process in the n−1th downlink subframe of the victimcell, and the CRS of the PDCCH region (or the CRS of an available OFDMsymbol) may be used for the measurement process in the n+1th downlinksubframe of the victim cell.

In the examples shown in FIG. 14 to FIG. 18, the downlink measurementmay include all of the measurement for RLM, CSI measurement,interference measurement, RRM measurement, and so on.

Signaling Downlink Measurement Resource Designation

Hereinafter, description will be made in detail on a signaling methodthat can be applied to the above-described various methods, such asdesignating a resource on which the downlink measurement is performed,in case of an inter-cell interference coordination (ICIC). The basestation may notify the user equipment of information indicating theresource (time resource, frequency resource, and/or space resource)area, which is used for the downlink measurement of the user equipment,by performing physical layer signaling or higher-level layer signaling.

For example, an RRCConnectionReconfiguration message, which is definedin the conventional 3GPP LTE standard document (e.g., TS36.331), may beconsidered to be used as a message for signaling the measurementprocess, which is performed by the user equipment. As shown in FIG. 22,measurement objects may be signaled through a measConfig informationelement (IE) included in the presently definedRRCConnectionReconfiguration message.

Referring to FIG. 22, according to the definitions in the current LTEstandard, the user equipment may be informed of the measurementinformation through the MeasObjectEUTRA IE. According to the current LTEstandard, since measurement is performed on a full bandwidth of eachcell, an allowedMeasBandwidth within the MeasObjectEUTRA is defined tohave a full RB signaled for each bandwidth (e.g., 6RB for a 1.4 MHzbandwidth, 15RB for a 3 MHz bandwidth, etc.).

According to what is proposed in the present invention, in the aspect ofinter-cell interference coordination (ICIC), in order to have the userequipment measure channel quality by using only a portion of thedownlink resource (e.g., a portion of the RB), additional information isrequired to be defined in the MeasObjectEUTRA IE, which is defined in aconventional RRC message.

For example, a field having the size of 1 bit may be added, in casePDCCH/PDSCH is differentiated and designated as the resource on whichdownlink measurement is to be performed, or in case 1^(st) slot/2^(nd)slot is differentiated and designated as the resource on which downlinkmeasurement is to be performed. Also, in case a specific RB isdesignated as the resource on which downlink measurement is to beperformed, a bit field may be configured in accordance with a signalingmethod using a bitmap, a signaling method performed by bundling multipleRBs, a signaling method directly signaling a starting point and anending point of an RB, or a signaling method using an offset value of astarting RB index and an offset value of an ending RB index. Similarly,a field designating a specific antenna port or OFDM symbol as theresource, on which the downlink measurement is to be performed, may alsobe configured. As described above, a signaling method for directing aresource, on which downlink measurement is to be performed, to a userequipment may be configured independently or in combination.

Hereinafter, description will be made on yet another signaling methodthat can be applied to the above-described various methods ofdesignating a resource on which the downlink measurement is performed,in case of an inter-cell interference coordination (ICIC).

The interfering cell may signal to the victim cell whether its subframeconfiguration corresponds to a normal subframe or an ABS (herein, ABSincludes ABS-with-MBSFN) by using a bitmap method. Herein, the bitmapmay be signaled in the form of a combination of 2 bitmaps (1^(st) bitmapand 2^(nd) bitmap). The 1^(st) bitmap corresponds to a bitmap notifyingwhich subframe is being configured as an ABS, and the 1^(st) bitmap alsoperforms a function of notifying which subframe can be converted to anormal subframe in a later process. The 2^(nd) bitmap corresponds to abitmap notifying which subframe is to be used for the measurement in thevictim cell, and the 2^(nd) bitmap may be configured as a subset of the1^(st) bitmap. It is proposed in the present invention that theinterfering cell should restrict the subframe being signaled through the2^(nd) bitmap, i.e., the subframe that is to be used for the measurementof the user equipment, which belongs to the victim cell, to anABS-with-MBSFN subframe (i.e., a subframe transmitting only the CRS ofthe control region). This indicates that the 2^(nd) bitmap consists ofsubsets of all subframes being configured as ABS-with-MBSFN subframes bythe interfering cell.

In order to perform a flexible measurement, the user equipment may beset to perform measurement only in a subframe, which is designated asthe ABS-with-MBSFN subframe by the interfering cell. In this case, byusing the signaling method that enables downlink measurement to beperformed only on the above-described specific resource, measurement ofa region having CRS interference existing therein and measurement of aregion having no CRS interference existing therein may be flexiblyapplied, whenever required, based upon the decision of the serving cell(victim cell).

Designating Resources for RRM Measurement

The measurement for RRM may be classified, for example, as ReferenceSignal Received Power (RSRP), Reference Signal Received Quality (RSRQ),and so on. Herein, the RSRQ may be measured by a combination of an RSRPand an E-UTRA Carrier Received Signal Strength Indicator (RSSI).

Hereinafter, a method for applying the above-described variousembodiments of measurement resource designation (or measurementrestriction) to RRM measurement will be described in detail.

In the conventional 3GPP LTE standard document (e.g., TS36.214), it isdefined that “E-UTR A Carrier Received Signal Strength Indicator (RSSI),comprises the linear average of the total received power (in [W])observed only in OFDM symbols containing reference symbols for antennaport 0, in the measurement bandwidth, over N number of resource blocksby the UE from all sources, including co-channel serving and non-servingcells, adjacent channel interference, thermal noise etc.” In otherwords, the power of an OFDM symbol, wherein the CRS for antenna port 0is transmitted, may be referred to as RSSI.

As described above, as an enhanced ICIC (eICIC) method, a method ofhaving the interfering cell configure an ABS or ABS-with-MBSFN subframe,in order to reduce the influence of a dominant interference, and havingthe victim cell perform measurement and/or transmission in thecorresponding subframe may be applied. This corresponds to an example ofan inter-cell interference coordination method using a time-divisionmultiplexing (TDM) scheme.

However, the solution using the above-described TDM scheme isdisadvantageous in that the dominant interference affects only aspecific resource element (RE) or a specific OFDM symbol. Also,depending upon which subframe is configured by the interfering cell asthe ABS subframe or as the ABS-with-MBSFN subframe in order to reduceinterference, the amount (or level) of interference may be largelyvaried. Considering the fact that the eICIC method is used to avoid theinfluence of a dominant interference on the victim cell, it ispreferable to perform a measurement having no interference in asubframe, wherein the solution using the above-described TDM scheme isapplied. Furthermore, it is important to perform measurement so that aconsistent interference level can be maintained in order to facilitatecompensations made by the base station.

FIG. 19 illustrates a change in amount of interference according tointerference cell subframe settings. For example, in the example shownin FIG. 19, the interfering cell may correspond to a macro base station,and the victim cell may correspond to a pico base station. However, thepresent invention will not be limited to the example given herein. Morespecifically, the same principle may be applied to two random cellsexchanging interference to and from one another, which will hereinafterbe described in detail.

As shown in the example of FIG. 19(a) (although the example shows a casewhere the CRS of the victim cell collides with the CRS of an aggressorcell, it will be apparent that the present invention can be applied evenin a case where there is no CRS collision), in case the subframe of theinterfering cell is configured as an ABS subframe, all CRSs of thevictim cell are influenced by the interference. Meanwhile, as shown inthe example of FIG. 19(b), in case the subframe of the interfering cellis configured as an ABS-with-MBSFN subframe, only the CRSs included inthe control region of the victim cell are influenced by theinterference. Therefore, the amount of interference is largely increasedin the case where the subframe of the interfering cell is configured asan ABS subframe (FIG. 1(a)) as compared to the case where the subframeof the interfering cell is configured as an ABS-with-MBSFN subframe(FIG. 19(b)). Furthermore, it will be apparent that the increase ininterference with respect to a change in the subframe configuration ofthe interfering cell is concentrated in a specific RE or a specific OFDMsymbol.

As described above, since the interference amount respective to thevictim cell can be largely reduced when the downlink subframe of theinterfering cell is configured as an ABS-with-MBSFN subframe, theperformance of the victim cell may be enhanced. However, since arestriction in the MBSFN configuration exists (e.g., in one radio frame,subframes of subframe indexes 0, 4, 5, and 9 cannot be configured asMBSFN subframes), all of the subframe cannot always be used as MBSFNsubframes. Therefore, in an ABS pattern, which is signaled by theinterfering cell to the victim cell ABS subframes may co-exist withABS-with-MBSFN subframes. And, in this case, due to a fluctuation ininterference, a problem of decreased measurement accuracy andimprecision in measurement may occur.

In order to resolve the above-described problem, a method of having thebase station designate and signal a resource, with respect to whichdownlink measurement of the user equipment is to be performed, will nowbe described. The embodiments of the present invention may be applied,to both a measurement using REs to which CRS is transmitted and ameasurement on OFDM symbols. Furthermore, in the following description,the downlink measurement includes all of CSI measurement, interferencemeasurement, measurement for RLM, RRM measurement (measurements of RSRP,RSSI, etc.).

For example, the victim cell enables the victim user equipment toperform downlink measurement only in a subframe configured as anABS-with-MBSFN subframe by the interfering cell, and, at this point, thevictim cell may designate and signal the measurement to be performedonly in the data region (PDSCH region) of the downlink subframe.Accordingly, since the downlink measurement of the victim cell can beperformed in a region where only the Null RE is transmitted (i.e., wherenothing is transmitted) from the interfering cell, the influence of thedominant interference caused by the interfering cell may be eliminated.In other words, a downlink resource of a victim cell, which is notsubstantially influenced by the interfering cell, may be designated andsignaled as the downlink measurement object of the victim cell.

As another example, in case the subframes of the interfering cell areconfigured as ABS subframes co-existing with ABS-with-MBSFN subframes,in order to allow the corresponding subframes to all be used for thedownlink measurement, the victim user equipment may designate and signalthe downlink measurement to be performed only with respect to OFDMsymbols having a constant interference amount, among the correspondingsubframes. This method may be usefully used in RSSI measurement, whereinOFDM symbol power is measured. For example, referring to FIG. 19, theOFDM symbols maintaining a constant interference level, regardless ofthe subframe configuration of the interfering cell, correspond to OFDMsymbol indexes 0, 1, 2, 3, 5, 6, 9, 10, 12, and 13 of the downlinksubframe. Therefore, when the base station designates and signals thedownlink measurement (e.g., RSSI measurement) to be performed by theuser equipment by using only the corresponding OFDM symbols (OFDM symbolindexes 0, 1, 2, 3, 5, 6, 9, 10, 12, and 13), the measurement result maymaintain a constant interference level regardless of the subframeconfiguration of the interfering cell. Also, the base station maydesignate and signal the downlink measurement to be performed by theuser equipment by using only a portion (one or more) of thecorresponding OFDM symbols (OFDM symbol indexes 0, 1, 2, 3, 5, 6, 9, 10,12, and 13). In other words, downlink resources of the victim cell,which receive constant interference from the interfering cell, may bedesignated and signaled as downlink measurement objects of the victimuse equipment.

FIG. 20 illustrates a downlink measurement method of a user equipmentwith respect to an inter-cell interference coordination and a method forsupporting the same according, to an embodiment of the presentinvention. Steps S2010, S2020, S2030, and S2050 may correspond todetailed process steps of a method for supporting downlink measurementof the user equipment in a 1^(st) cell, and steps S2030, S2040, andS2050 may correspond to detailed process steps of a method forperforming downlink measurement of the user equipment in the 1^(st)cell. Hereinafter, each process step will be described in detail.

In the downlink measuring method and the measurement supporting method,which are described with reference to FIG. 20, it is assumed that 2cells, i.e., a 1st cell and a 2nd cell, exchanging interference exist.Hereinafter, in the description of the present invention, it will beassumed that the 1st cell (or 1st base station) corresponds to thevictim cell, and that the 2nd cell (or 2nd base station) corresponds tothe interfering cell. Additionally, it will also be assumed that theuser equipment corresponds to a victim user equipment being served bythe 1st cell (victim cell) and being influenced by an interferencecaused by the 2nd cell (interfering cell).

In step S2010, the 1st cell may receive information on the downlinksubframe configuration of the 2nd cell from the 2nd cell. Alternatively,in case the 2nd cell configures the downlink subframe in accordance witha pre-decided pattern, the 1st cell may implicitly acquire downlinkconfiguration information of the 2nd cell without having to receive thecorresponding information directly from the 2nd cell. Herein, thedownlink subframe configuration information of the 2nd cell maycorrespond to information indicating whether each downlink subframe ofthe 2nd cell corresponds to a normal subframe, an ABS subframe, an MBSFNsubframe, or an ABS-with-MBSFN subframe. Furthermore, the 1st cell maybe informed of a downlink subframe timing of the 2nd cell through thedownlink subframe configuration of the 2nd cell. Accordingly, the 1stcell may be informed of an offset degree by which downlink subframeboundaries of the 1st cell and the 2nd cell are shifted. As a result,among its own downlink resources (time, frequency, and/or spaceresources), the 1 st cell may be aware of the resource that is beinginfluenced by the interference caused from the 2nd cell.

In step S2020, based upon the information on the downlink subframeconfiguration of the 2nd cell, the 1st cell may determine the resourceson which measurement is to be performed by the user equipment, i.e., themeasurement objects, among the downlink resources of the 1st cellitself. More specifically, in deciding the measurement objects, the 1stcell may consider resource areas that are influenced by the interferenceoccurring from the 2nd cell. For example, the 1st cell may determineresources that are not substantially actually influenced by theinterference caused from the 2nd cell as the measurement objects.Alternatively, the 1st cell may also decide resources that experienceconstant interference from the 2nd cell as the measurement objects.

In step S2030, the base station may transmit information on themeasurement objects to the user equipment. The information on themeasurement objects may correspond to information specifying downlinkresources of the 1st cell, which are designated to be measured by theuser equipment. And, the information on the measurement objects may betransmitted to the user equipment via, for example, the RRC signaling.Information on the measurement objects may be expressed as a combinationof at least one set of time resource information, for example,information indicating a certain downlink subframe, whether the resourcecorresponds to the control region or the data region, a certain slot, acertain OFDM symbol, and so on. Additionally, the information on themeasurement objects may also be expressed as a combination of at leastone of information on the time resource, information on the frequencyresource (the corresponding RB), and information on the space resource(the corresponding antenna port). Accordingly, the 1st cell may notifythe resource on which the user equipment is to perform downlinkmeasurement in RE units.

In step S2040, the user equipment may perform measurement on themeasurement objects that are designated by the 1st cell. The measurementperformed by the user equipment may include all of the measurement forRLM, the measurement for CSI reporting, the measurement of interference,and the RRM measurement (measurement of RSRP, RSSI, and so on). Herein,the 1st cell may designate the measurement that is to be performed bythe user equipment.

In step S2050, the user equipment may report the measurement result tothe 1st cell. Also, by specifying the downlink resource on which the 1stcell and the user equipment actually perform communication, only themeasurement result on the corresponding resources may be taken intoconsideration. Therefore, a more accurate and efficient measurementresult may be provided, and unnecessary RLF may also be prevented.Accordingly, as compared to a case where inter-cell interference causesa large influence, measurement results, such as channel quality, may bemore efficiently used.

In the downlink measurement method and the method for supportingmeasurement according to the present invention, which are described withreference to FIG. 20, details of the above-described various embodimentsof the present invention may be independently applied or 2 or moreembodiments may be applied at the same time. And, in this case,overlapping details will be omitted from the description for simplicityand clarity.

FIG. 21 illustrates a base station device 2110 and a user equipmentdevice 2120 according to a preferred embodiment of the presentinvention.

Referring to FIG. 21, the base station device 2110 according to thepresent invention may include a reception module 2111, a transmissionmodule 2112, a processor 2113, a memory 2114, and a plurality ofantennae 2115. The plurality of antennae 2115 indicates that the basestation device supports MIMO transmission and reception. The receptionmodule 2111 may receive various signals, data, and information on anuplink from the user equipment. The transmission module 2112 maytransmit various signals, data, and information on a downlink to theuser equipment. The processor 2113 may control the overall operations ofthe base station device 2110.

The base station device 2110 according to the embodiment of the presentinvention may be configured to support measurement of the user equipmentdevice. The base station device 2110 may correspond to a base stationthat is influenced by an interference caused from another base station.The processor 2113 of the base station 2110 may be configured to acquireinformation on downlink subframe configuration of the base stationcausing the interference. Also, the processor 2113 may be configured todecide measurement objects among the downlink resources of the basestation device 2110, based upon the downlink subframe configuration ofthe base station causing the interference. Additionally, the processor2113 may be configured to transmit information on the decidedmeasurement objects to the user equipment 2120 through the transmissionmodule 2112. Furthermore, the processor 2113 may be configured toreceive measurement results respective to the measurement objects fromthe user equipment 2120 through the reception module 2111.

Moreover, the processor 2113 of the base station device 2110 performs acalculation/operation process on information received by the basestation device 2110, information that are to be transmitted outside, andso on. The memory 2114 may store the operated information for apredetermined period of time, and the memory 2114 may be replaced byanother element, such as a buffer (not shown).

Referring to FIG. 21, the user equipment device 2120 according to thepresent invention may include a reception module 2121, a transmissionmodule 2122, a processor 2123, a memory 2124, and a plurality ofantennae 2125. The plurality of antennae 2125 indicates that the userequipment device supports MIMO transmission and reception. The receptionmodule 2121 may receive various signals, data, and information on adownlink from the base station. The transmission module 2122 maytransmit various signals, data, and information on an uplink to the basestation. The processor 2123 may control the overall operations of theuser equipment device 2120.

The user equipment device 2120 according to the embodiment of thepresent invention may be configured to perform measurement on a downlinkfrom the base station device 2110. The user equipment device 2120 maycorrespond to a user equipment that is influenced by an interferencecaused from another base station. The processor 2123 of the userequipment device 2120 may be configured to receive information onmeasurement objects from the base station device 2110 through thereception module 2121. Also, the processor 2123 may be configured toperform measurement on the designated measurement and to transmit themeasured results to the base station device 2110 through thetransmission module 2122. Herein, the measurement objects may be decidedamong downlink resources of the base station device 2110, based upon thedownlink subframe configurations of another base station causinginterference.

Moreover, the processor 2123 of the user equipment device 2120 performsa calculation/operation process on information received by the userequipment device 2120, information that are to be transmitted outside,and so on. The memory 2124 may store the operated information for apredetermined period of time, and the memory 2124 may be replaced byanother element, such as a buffer (not shown).

In the above-described detailed configuration of the base station device2110 and the user equipment device 2120, details of the above-describedvarious embodiments of the present invention may be independentlyapplied or 2 or more embodiments may be applied at the same time. And,in this case, overlapping details will be omitted from the descriptionfor simplicity and clarity.

Furthermore, the description of the base station device 2110 of FIG. 21may also be equally applied to a relay station device functioning as adownlink transmission subject or an uplink reception subject. And, thedescription of the user equipment device 2120 of FIG. 21 may also beequally applied to a relay station device functioning as an uplinktransmission subject or a downlink reception subject.

The above-described embodiments of the present invention can beimplemented by a variety of means, for example, hardware, firmware,software, or a combination of them.

In the case of implementing the present invention by hardware, thepresent invention can be implemented with application specificintegrated circuits (ASICs), Digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), a processor, a controller, amicrocontroller, a microprocessor, etc.

If operations or functions of the present invention are implemented byfirmware or software, the present invention can be implemented in theform of a variety of formats, for example, modules, procedures,functions, etc. The software codes may be stored in a memory unit sothat it can be driven by a processor. The memory unit is located insideor outside of the processor, so that it can communicate with theaforementioned processor via a variety of well-known parts.

The detailed description of the exemplary embodiments of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the exemplary embodiments, those skilled in the artwill appreciate that various modifications and variations can be made inthe present invention without departing from the spirit or scope of theinvention described in the appended claims. For example, those skilledin the art may use each construction described in the above embodimentsin combination with each other. Accordingly, the invention should not belimited to the specific embodiments described herein, but should beaccorded the broadest scope consistent with the principles and novelfeatures disclosed herein.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above exemplary embodiments are therefore to beconstrued in all aspects as illustrative and not restrictive. The scopeof the invention should be determined by the appended claims and theirlegal equivalents, not by the above description, and all changes comingwithin the meaning and equivalency range of the appended claims areintended to be embraced therein. Also, it will be obvious to thoseskilled in the art that claims that are not explicitly cited in theappended claims may be presented in combination as an exemplaryembodiment of the present invention or included as a new claim bysubsequent amendment after the application is filed.

The embodiments of the present invention are applicable to variousmobile communication systems.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method for performing a measurement by a userequipment (UE) in a wireless communication system, the methodcomprising: receiving, from a first base station, information indicatinga resource region for performing the measurement; performing themeasurement over the resource region; and transmitting, to the firstbase station, a report for the measurement, wherein the resource regionis determined as a combination of at least one orthogonal frequencydivision multiplexing (OFDM) symbol and at least one downlink subframe,and wherein the resource region is determined by the first base stationbased on information indicating at least one subframe configured as analmost blank subframe (ABS) by a second base station.
 2. The methodaccording to claim 1, wherein the measurement is performed over all OFDMsymbols in the at least one downlink subframe.
 3. The method accordingto claim 1, wherein the information is received through radio resourcecontrol (RRC) signaling.
 4. The method according to claim 1, wherein theinformation indicating the at least one subframe configured as the ABSis received by the first base station from the second base station. 5.The method according to claim 1, wherein the measurement is forreference signal received quality (RSRQ), and the RSRQ is measured byreference signal received power (RSRP) and a received signal strengthindicator (RSSI).
 6. A method for supporting a measurement of a userequipment (UE) by a first base station in a wireless communicationsystem, the method comprising: receiving, from a second base station,downlink subframe configuration information indicating at least onesubframe configured as an almost blank subframe (ABS) by the second basestation; transmitting, to the UE, information indicating a resourceregion for performing the measurement, determined based on the downlinksubframe configuration information; and receiving, from the UE, a reportfor the measurement performed over the resource region, wherein theresource region is determined as a combination of at least oneorthogonal frequency division multiplexing (OFDM) symbol and at leastone downlink subframe.
 7. The method according to claim 6, wherein themeasurement is performed over all OFDM symbols in the at least onedownlink subframe.
 8. The method according to claim 6, wherein theinformation indicating the resource region is transmitted through radioresource control (RRC) signaling.
 9. The method according to claim 6,wherein the measurement is for reference signal received quality (RSRQ),and the RSRQ is measured by reference signal received power (RSRP) and areceived signal strength indicator (RSSI).
 10. A user equipment (UE) forperforming a measurement, the UE comprising: a reception module; atransmission module; and a processor configured to: receive, from afirst base station, information indicating a resource region forperforming the measurement, through the reception module, perform themeasurement over the resource region, and transmit, to the first basestation, a report for the measurement, through the transmission module,wherein the resource region is determined as a combination of at leastone orthogonal frequency division multiplexing (OFDM) symbol and atleast one downlink subframe, and wherein the resource region isdetermined by the first base station based on information indicating atleast one subframe configured as an almost blank subframe (ABS) by asecond base station.
 11. A first base station for supporting ameasurement of a user equipment (UE), the first base station comprising:a reception module; a transmission module; and a processor configuredto: receive, from a second base station, downlink subframe configurationinformation indicating at least one subframe configured as an almostblank subframe (ABS) by the second base station, through the receptionmodule, transmit, to the UE, information indicating a resource regionfor performing the measurement, determined based on the downlinksubframe configuration information, through the transmission module, andreceive, from the UE, a report for the measurement performed over theresource region, through the reception module, wherein the resourceregion is determined as a combination of at least one orthogonalfrequency division multiplexing (OFDM) symbol and at least one downlinksubframe.